BW transparencies in general look best if your film has (after the reversal development) a very low Dmin value, and a very high Dmax value.
In most cases you get this using films with a clear base. The following films all have a clear base and deliver very good to excellent results reversal processed (in alphabetical order):
Adox CMS 20 II
Agfa Scala 200X
Agfa Copex Rapid
Ilford Delta 100 (120)
Rollei Retro 80S
Rollei Superpan 200
Rollei Retro 400S
As you are living in the UK, the most convenient option for you is probably sending your films to one of the 5 BW reversal labs in Germany (Germany is the "paradise for slide film shooters", in no other market reversal film is so popular).
I can recommend Photostudio 13 www.photostudio13.de (they are developing all the above mentioned films in the [modified] Scala reversal process) and
Klaus Wehner www.schwarz-weiss-dia.de (he has designed his own high-end 20-step reversal process).
Both labs do international business / shipping.
I second that. Photostudio13 does a magnificent job. It's also cheap comparing to other labs.
Another option is the Foma reversal kit.