118.825 is the sector Clippy (Milton Keynes) to BPK.
Have a look in the En-Route section of the AIP.
Thames Radar 132:700 would help you too.
London Info isn't going to help too much. They have a massive area to cover and won't give you a radar service.
Ring your local airfield that would handles IFR traffic and ask them. Cambridge, for example, would give you a climb to the base of CAS and track to BKY. Essex Radar then take you and cive you a further climb and headings / own nav to join CAS.
If you were departing from a non-IFR airfield, you would have to self position to the CAS joining point under a traffic / deconf service from your local LARS proivider. Farnborough now have a good service. Look up the local chart in the Jeps or via NATS
NATS | AIS - Home
http://www.ead.eurocontrol.int/eadba...2010-06-03.pdf