The problem is that antibody testing is likely to give a pessimistic view of the situation, there are, I beleive likely to be people who would test negative for antibodies that have had it and are immune as it seems they might fade to undetctable levels quickly, but the memory of how to produce them is retained, there is a lot starting to come out about T cell immunity now.
So yes I'd have the test, but I think looking at the numbers of positives across the country as a whole will give a misleading picture, rather like this R rate, yes, it probably was an average of 3, but an average isn't all that useful, it could have been 0.6 in most offices, 0.8 in supermarkets, 3.2 on the tube and 28 in nightclubs for instance, the threashold for herd immunity is going to be therefore similarly spit, you don't need 60% across the whole country, you need a very high number of those likely to frequent nightclubs, and much lower in the elderly populations who only go to the shop and the post office, its probably not massivly far away. Of course there will be outbreaks when it is 'seeded' into environments where it can easily spread but these seem to rely on quite specific circumstances, we have seen that in meat packing factories and cramped textile manufacturers (which had a more ethnically subseptable population)