All these orchestras which welcome intermediate string players (and hundreds more orchestras and bands) are listed on the excellent Amateur Orchestras' Web site which I've mentioned before. It's run by one enthusiast and entries are loosely divided into regions. Quotations are from the orchestras' entries on the AO Web site.
http://www.amateurorchestras.org.uk/

I've just heard about the first of these, due to start next term. All three have contact links from the AO site or their own.

West London Strings (NEW)
"A new group starting in September 2009. No auditions; Grade 3 upwards welcome. Age 18 to 80+. Music will be chosen and arranged to suit the membership. Our main aim is to experience the enjoyment of working at worthwhile music." Conductor: Robin Wedderburn. Mondays, St Marys Church, Edith Road, West Kensington, London W14 9BA (near Hammersmith Flyover)
http://www.westlondonstrings.org.uk/

Windsor Community Orchestra
"A local orchestra for all ages, including children above 9 years old, young people and adults, who meet to learn a wide range of music and perform together in the community. We welcome players of any grade or ability, but the orchestra is designed for those under grade 5." Conductor: Elaine Foakes. Oakfield First School in Imperial Road, Windsor on Thursdays from 7.00 to 9.00 p.m
http://www.windsororchestra.org.uk/

East London Late Starters' String orchestra
Intermediate Performance Ensemble
"This friendly and supportive string orchestra is for players who have been playing for at least a year or so and are looking to develop their ensemble playing skills. Ten week term with opportunities to perform at ELLSO concerts."
Conductor: Nick Coote. Monday Evenings 7.15-9.30pm Bow Mission, Merchant Street E3 4LY
http://www.ellso.org.uk/

As the beginning of the new term for most English amateur groups is getting near I hope no-one will mind if I push the link to the Amateur Orchestras' (and Bands') Web site and my little list of opportunities in this thread back into view! Best wishes for a happy term's music-making everyone.