But unless it's heated, with the UK climate you can only really use a swimming pool for a few days a year, whereas a hot tub you can use even if it's below 0 and snowing
We used to have a proper 6' deep, 20' long and 10' wide pool at my grandmothers old house (quick look at Google maps suggests they've built over it) and it was amazing. We'd set it up from about May/June and probably empty it late September early October. Took a good few days to fill up. Once my Grandmother was out of the country for a month or so and we kept/forgot the heating on it for a few days and it was so warm, was like a lukewarm bath! Loved going for late night swims with the lights on.
But for the rest of the year it was pretty unusable! Was a lot of hard work getting it ready though, draining it, cleaning the tiles, clearing out all the frogs/toads who decided to lay spawn in there