Should be ok if replacing a fillament bulb in original position so as not to show a white light to the rear, and that they are white, and that they have an approval mark. To quote -
"(i)a marking designated as an approval mark by regulation 5 of the Designation of Approval Marks Regulations and shown at item 10 of Schedule 4 to those Regulations; or
(ii)a marking designated as an approval mark by regulation 4 of the Designation of Approval Marks Regulations and shown at item 4 of Schedule 2 to those Regulations."
A lot of cheap LED lamps don't have this, as it costs to get them through approval etc., but you can get approved ones ('e' marked) for most applications. It's very unlikely that you would have someone check the bulb for markings, but if they did, it could mean 3 points on your licence for having the 'wrong' ones, even if they came from the same production line.
The regulations that apply are in the Road Vehicle Lighting Regulations 1989 (as ammended).
See RVLR1989