1) ATC unit Farnborough North on 132.8 on departure from EGLD, they'll look after you almost all the way, switching you to EAST as it becomes necessary - then (depending upon routing) Manston, Lydd or London info then direct L2K. Lille are rarely interested these days.
2) You have to file your flight plan (electronically) usually an hour before departure. In addition, you need to file a GAR form four hours before you intend to arrive back in the UK. Blank ones will be available from either TPC or DSF, whichever your leaving from. You can give these (or fax them) to the guys and gals in the tower before departure.
3) See above. Straight from Manston / London Info to L2K approach. Whoever you're working will will ask for an estimate that you'll reach the FIR boundary - then a few minutes before you reach it or when you report 'mid-channel' they'll send you en-route.
4) L2K approach will want you to report usually at Bologne Harbour if approaching from the North, but a direct routing DVR-ING would be fine. You can work out a cross cut from LYD to tell you when you're mid-channel.
If you need any further help, PM me.