The 2011 UK Census will have a facility for on-line completion of the census return, so it might be possible for you to participate from Bulgaria. I think that this would depend on you having some sort of "UK base" or address so that you would be included in the mailing of the instructions/forms (estimated to be sent to 25m households).
I worked as a Census Enumerator back in 1981, and even then it was being suggested that the Census be discontinued, as the government already has all the information that can be gained from the census sitting on a database somewhere. This is even more true today. I predict that the 2011 will be the last - partly because of cost, partly because of improved alternative sources of data, and partly because of the problem of getting a full picture - if it's so hard to get the under 40's to vote, what chance is there of getting them to fill out a census form?? Bad news for future genealogists.