There are many ice rinks in London, that provide 6-week adult courses, but if you say you are on a good level then its worth maybe taking private lessons, as you will learn more and improve at a more rapid rate.
I have private lessons at Queens Ice Rink, in Central London. Its a really nice rink, with great ice. Even though its quite small compared to some rinks, they do play good music and they have coloured lighting too - definietely worth a visit! I feel I have improved a lot in 4 weeks.
Hope this helps!
Nikki