I think that, in general, if the Sukhoi 26/29 is built after 1993, it is ineligible for a Permit to Fly in the UK because that is the date that has been "determined" as being the last year in which they qualified as being used by the military. So go for an earlier built aeroplane.
Yak 54 I have no idea, but suggest you contact Mark Jefferies at
www.yakuk.com
He knows rhyme and verse on those types.
Yak 55 will be eligible for a Permit to Fly provided built before 1993, which I think they all were. They are not C of A aeroplanes and do have "military" history as were flown by DOSAAF. Likewise, I think, the Sukhois.