I'm running my son in the Junior TKM 4 stroke class next year. The engine is bullet proof and quick and will do between 50-100 hours before a rebuild. It's all sealed and cheating is strictly monitored. You can probably pick up a complete outfit ready to go at this time of the year and at a really good price. The senior entry level is always pretty good.
Tyres are of a hard compound which will last you longer than those used by the Rotax classes.
The F6 club run a friendly championship based in the south of the country, where the races are on a Saturday.
Try
www.ukkarting.co.uk for a decent used outfit.