I can surely recommend John Howell, who is very experienced in international law and there is a Bulgarian attorney who is helping them for all Bulgarian cases, which is very important because not all UK solicitors understand what is happening on the local market and haven't got knowledge about Bulgarian law.
The best option for you would be independent English speaking Bulgarian attorney. If John Howell is too expensive for you then maybe you can take a look at: www.leg.bg (Bulgarian law portal)
If you need personal recommendation you can PM me and I will be glad to arrange for you a free initial consultation so you take a decision for yourself.
Having an independent lawyer with any property deal overseas is a decision that should not be taken lightly so make sure you have good solicitor !
Well you could try out, Convex. I do not know if they deal with over seas property purchase, but you could contact them to find out. I have dealt with them when I relocated to the suburbs and I found them rather professional.Maybe they can help you. Bye and all the best.