As someone who bought his own plane in less than 1 year of getting the PPL, and never regretted it for even 1 second, I think one can make a very good case for ownership, over self fly hire, etc. and the biggest pluses will not be financial (the strictly calculated financial breakeven point over SFH will indeed need a lot of hours to be flown annually). The biggest pluses will be access, maintenance to your standard, etc.
However I think buying a plane for hour building just seems doing it upside down. Flying 1000hrs (hrs which count towards an ATPL) in your own plane is going to cost you of the order of £100k, and where on earth are you going to be flying 1000hrs to? I have accumulated over 1100hrs in 8 years in the TB20 but I have gone to a lot of far away places in Europe, which swelled the logbook nicely, but one would not be doing trips like that in something which is cheap to fly. Sure one could buy a C152 and just fly it up and down the UK and log the flights (I wonder how many people have invented a few logbook entries; it would be so obviously easy if flying between strips, where no records are kept) but it would be excrutiatingly boring. And it would be a lot cheaper in the USA.
I think one needs to get into private flying for a good reason (touring, aerobatics, whatever) but not plain hour building.