Thomas,
Hope the resident controllers here don't mind a non-controller sticking his nose in! :=
You originally asked about flight outside controlled airspace. As others have said, although I don't think anyone's explicitly pointed out your mistake - Southampton is not outside controlled airspace, it is in its own Class D zone which is connected to the airways system.
However, now that this is resolved, are you still interested in knowing about flight outside controlled airspace? If so, read on:
Imagine, for a moment, that you were routing to Exeter, rather than either Southampon or Bournemouth. Exeter is outside controlled airspace, but it lies underneath an airway. So first of all, you'd have to decide between 2 options:
a) Route inside controlled airspace as much as possible. From Southampton, you'd probably route north to NORRY, then L9 to BCN, and south on N864. (Not sure if there's a better route off the top of my head, I don't have an airways chart to hand, but the idea would be the same for any other route.)
OR
b) Route airways to SAM, then direct to Exeter outside controlled airspace, which would be quite a few miles shorter.
If you went for option a), you'd find that Exeter's standard departures and arrivals join and leave the airways close to the airport. This is a well-established procedure, so you'd get an airways clearance on the ground prior to departure, and you'd be handed to Exeter Radar on leaving the airways. The only difference you'd notice from a flight wholly inside controlled airspace is that for a few short miles, you'd have to choose from the available services outside controlled airspace (e.g. Deconfliction Service or Traffic Service), and be particularly aware of the possibility of traffic which the controller is not aware of.
If you went for option b), then you'd really experience (and need to understand) flight outside controlled airspace, in which case the following pointers will be helpful:
1) ATC. You should have in mind a plan for who you would like to speak to. In practice, ATC will make a suggestion as to who you should speak to next when you reach the edge of their area, and might even pre-notify you to the next unit, but you don't have to follow their suggestions (do tell them if you're doing something other than what they suggest), and you may just get told to "Squawk 7000 and freecall en-route" if you're unlucky.
I'd suggest you choose who to speak to by going for the first one that's available on the following list:
a) A LARS station if below FL100, or a Middle Airspace Radar Service if above FL100 (see AIP ENR 1.6.3 and 1.6.4 (http://www.ead.eurocontrol.int/eadbasic/pamslight-ED14EE5E1BD1CA02F415796FC3F09C6A/7FE5QZZF3FXUS/EN/AIP/ENR/EG_ENR_1_6_en_2011-01-13.pdf) and AIP ENR 6.1.6.3 (http://www.ead.eurocontrol.int/eadbasic/pamslight-ED14EE5E1BD1CA02F415796FC3F09C6A/7FE5QZZF3FXUS/EN/Charts/ENR/NON_AIRAC/EG_ENR_6_1_6_3_en_2010-11-18.pdf))
b) An ATC unit that has radar
c) An ATC unit without radar (you won't get anything other than a Basic service), or
d) London Information (you won't get anything other than a Basic service)
For our theoretical route, on a weekday up until 5pm, and below FL100, you'd probably want to speak to Boscombe Radar (126.7) when you leave controlled airspace, then Yeovil Radar (127.35). After 5pm and on weekends, both those stations are closed, so you'd probably speak to Bournemouth Radar (119.475).
2) Choice of service. See AIP ENR 1.1.2 (http://www.ead.eurocontrol.int/eadbasic/pamslight-ED14EE5E1BD1CA02F415796FC3F09C6A/7FE5QZZF3FXUS/EN/AIP/ENR/EG_ENR_1_1_en_2011-01-13.pdf) for full details, but basically, when en-route, you have a choice of requesting Basic, Traffic or Deconfliction service.
Basic service does not include any traffic information unless the controller happens by chance to spot a conflict which he feels is important enough to mention to you. Generally not suitable for flight in IMC unless no other service is available.
Traffic service is the one I'd suggest going for. You will be identified on radar, and any conflicting traffic will be called out to you. You are responsible for avoiding the traffic. In practice, very often no avoidance is necessary, or a turn will take you away from the traffic, but if in doubt, upgrade temporarily to Deconfliction service
Deconfliction service includes not only calling traffic to you, but giving you instructions to avoid the traffic. The rules for how much separation you are given can be quite draconian, especially if the controller is not talking to the traffic concerned, and even more so if he isn't using a Mode C transponder, so you tend to get lots of vectors to avoid the guys who are VFR several thousand feet below you.....
3) Joining controlled airspace. On our imaginary route, you wouldn't need to concern yourself with this on the way to Exeter, but on the return leg, you'd need to get an entry into controlled airspace. You can pretty much ask any controller for a clearance - if you've departed Exeter and you want to join airspace at Southampton then either Boscombe or Bournemouth would be fine, but be sure to do it in plenty of time. In practice, they'll most likely pre-notify you to the appropriate controller, then hand you over before you reach the boundary of airspace. At this point you are not cleared in, and should still be prepared to hold if you don't get your clearance in time. But once you are two-way with the appropriate controller, he will hopefully issue you with a clearance fairly quickly. You may be cleared to enter at a particular level, in which case be sure to be level before joining (or tell the controller if you can't... he may be able to amend you clearance so you can join in the climb).
4) Flight plans. It is not generally possible to use DCT on a flight plan for a leg of more than 50nm (although there are exceptions). So ending your flight plan with "<airway> SAM DCT EX" would not work. Instead, try "<airway> SAM DCT YVL DCT EX". It is not necessary to actually route via YVL, it is only necessary to put it on the flight plan so that no single leg appears to be more than 50nm.
5) Level planning. You will need to fly at a level which complies with the Quadrantal Rule. Don't expect ATC to tell you what level to be at until you are approaching your destination. If you are high, you will probably want to have an idea of where to start descending. When changing level, advise ATC first - apart from being polite to tell them what you're doing, they may have traffic to affect.
Hope that's of some help (or at least general interest), if not for this flight then for a future one! And I'm looking forward to the controllers on the forum correcting me on the finer points now.....!
FFF
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