To what extent could the British state become a threat to socialists? The simple answer is that I don’t know.
My rough guess would be this: the security apparatus monitors anybody who either in opinion or deed questions the existing power structure in Britain. Monitoring of the internet is ubiquitous; and no doubt all our names along with other details are logged.
Yet the number of people who roughly agree with what we say could be counted in the millions, but only very occasionally do they articulate their thoughts on the net or elsewhere. But that still leaves a few thousands of people who do comment or act regularly and who also urge others to do so. It is these people who are identified on security lists.
Britain, though, is not about to throw liberal democracy overboard wholesale and cull its left-wing critics, as per Indonesia 1965 or Chile 1973. Therefore, I see little personal danger. The only adverse consequence might occur in the unlikely event of anybody getting near a ‘sensitive’ position in the British state; they would be blacklisted immediately. I for one can live with that.