Backing up Aiglons post, for a more informative answer contact RGA for the Sukhoi's and Yak Uk (
www.yakuk.co.uk)
for info on the Yak's.
As the Yak 54 is advertised as being type certificated it should be acceptable for a UK C of A, but as no examples of this aircraft have been placed on the UK register yet it may be a lengthy job to get the paperwork sorted out.
The Yak 55 is now on the Uk register but I do not know if that is on a Permit to Fly or C of A. (probably a C of A as I cannot see the aircraft as being ex military!)
Whatever you decide be sure to ask questions regarding maintenance requirements, particularly if there are any airframe life and overhaul life stipulations such as the ones the Yak 52 has run into recently.
Regards