Normal practice if you want minimal agro is to contact the nearest LARS provider (there's a map showing coverage and frequencies in the AIP or any of the UK VFR flight guides such as Pooleys) and ask for a FIS. They may or may not then give you a squawk code (as often as not they're perfectly happy with 7000C), and then generally ignore you unless you inadvertently enter their airspace or (if they're in a good mood) there's known traffic in your vicinity.
Typical calls (not intending to teach you to suck eggs, but standard phraseology does vary between countries, regardless of ICAO's best efforts) would be something like:
- Luton RADAR, this is G-ABCD
- G-ABCD, Luton Radar, pass your message.
- Luton Radar, G-ABCD is an A109 Helicopter, Brussels to Coventry, routing ..., 1500ft, request flight information
- G-ABCD, flight information service, squawk ****, QNH 10**, remain clear of controlled airspace, report passing_ _ _ _
- G-ABCD, flight information, 10**, squawking ****, report _ _ _
Or thereabouts. Don't actually expect traffic information, regard it as a bonus if it happens, just treat it as a way of oiling the ATC wheels.
G