Thinking of racing? Here's what's required

https://www.britishcycling.org.uk/road/article/bcst_rulebook_and_constitution?c=EN

GENERAL, ROAD AND TRACK TECHNICAL REGULATIONS

Section 8

8.1.2 When competing under these Regulations riders
(other than as provided in T.R. 8.1.8 and 8.1.9)
shall wear Club or Region registered design and
colours, as is appropriate.

8.1.9 In Road and Track events, Youth, Fourth and
Provisional category licence holders and BC
non licence holders will be allowed to wear
any suitable clothing whilst competing in
events under the Technical Regulations of the
Federation, including that bearing registered or
unregistered advertising except in the case of
National Championships when they must comply
with T.R. 8.1.1 to 8.1.6. However protective
headgear must be worn in accordance with T.R.
8.6.