1. As far as I am aware Shoreham is not PPR,
2. As far as I am aware Shoreham will always give you an approach service if you arrive IMC within published hours and conditions are "prorperly" IMC - but if you pitch up for effectively a training IFR arrival you should book a slot,
3. This concept of filing a FP for an IFR flight outside class A in the UK is nuts - it simply will not smooth your passage through class D and as bookworm says the chances are no one will take any notice of the FP other than your destination (and that includes class D); sorry but I think you are simply wasting your time but file as you will,
4. Yes, correct a FP is required TO or FROM the UK.
Problem with that is that I've had to freecall every en route frequency, increasing workload except where the guys on the ground have provided a handover out of courtesy. Would it be sensible to file a flight plan with all the en route ATS units even when staying outside class A?
See above and no. You will not get any more preference because you have filed than if you had not. For example take a flight Goucester, Brize, Shoreham. The service Gloucester can provide is a basic service and thats the most you will get. Brize can and will give you a traffic service as soon as you ask and will give you an IFR class D transit. They may or may not hand you over to Farnborough but it will make no different whether you have filed or not. You are more likely to get a handover if you indicate you are IMC IFR and ask if they can arrange a handover to Farnborough please. In these conditions when most people are on the ground and Farnborough not too busy they will organise the handover. Finally you could ask Farnborough to arrange a handover to Shoreham - but why? There is nothing to handover as all Shoreham can offer is a basic service. Even in the procedure at Shoreham the procedure is entirely outside CAS.
The last thing a busy controller wants to sequence at the last minute is you while you are submitting all of the info on a FPL over the radio.
.. but sorry all what info? Lets say you wish to transit Solent West to East on track Shoreham, somewhere East of Bournemouth (outside CAS) you will tell them you are G-XXXX, FL75 requesting IFR transit direct Shoreham - that is your FP and if you had filed, the length of the call would be no different, in fact it could end up being exactly the same call. With any luck the controller will give you a squawk, QNH and clearance direct on track Shoreham. He might ask you to route via SAM, give you a change of level or ask if you can accept vectors but short of refusing your transit thats about it.
Frankly, although some ask, these guys are really not interested in your life history, they dont want to know where you have come from, and they dont want to know where you are going, they really dont want to know whether you are 1, 2 or 3 pax and whether you are expecting SAM at 15, 25 or 35 past! All they want to know is how you wish to transit THEIR class D - its not London info.
I hasten to add that I am only delaing with flights outside class A - as I said earlier it all changes if you want to go airways and of course you can enjoy the luxury of all the handovers etc and clearly must file but dont forget that en route Shoreham you will more than likely get sent literally round the houses whatever you file and might find they get shot of you (so far as class A is concerned) at the earliest opportunity.