Hi FlyDirect
In direct answer to your questions here are my suggestions...
1. The EET at the FIR boundary is enough. A VFR flight plan is more or less a formality, as long as you are not hours outside of the ETA at the edge of FIR there won't be too many problems. And besides once the flight plan is activated the EET is calculated from the departure aerodrome airborne time. The actual time you get to the FIR boundary wont be too far from the EET.
2. I'd keep the ATC agencies simple. On the UK charts there are the London/Scottish FIS frequencies and the same thing exists on charts of foreign areas too. Its a good place to begin, however bare in mind that you may be flying directly into CAS and will need to speak to the controlling authority of that airspace. Some guidance on our neighbours (France, Belguim and the like) is in the Pooleys flight guide I believe. This also ties in with question 3.
3. Getting to the FIR boundary is done in the usual way. In my experience (which is limited in all honesty) whoever you are speaking to outbound, Manston for example, will ask you to report at a specfic place; Manston may use 'report mid channel'. When you do report mid channel they usually provide you with a frequency to change to. Although be prepared for them not to and tell you to 'change enroute'. A channel crossing into France from the south east will usually involve speaking to Lille Information, can't remember the freq off hand. Also consider that where the UK uses a Basic Service, European ATC will use Flight Information Service - for all intents and purposes its the same thing, just worded differently - again I believe there's guidance in the Pooleys guide, and on the AIS website in the AIP.
There are people here that are seasoned pros at this type of trip who can advise you better, but I hope this helps as a starter.
GW