Don't do it @Newhousename someone I know had an outdoor swimming pool put in several years ago, and it cost about £70K, and it's been nothing but a PITA. Hardly used really. Even last summer when it was really hot, they found that some of the time it was hard to swim in it/stay in it, as it was nearly 40C and the sun was blaring down on them. Also, as has been said, high maintenance, and costly too. This person has been talking about how it's more bother than it is fun, but they can't bring themselves to fill it in as it cost so much money!
There used to be a big outdoor pool (lido) in a town that's not a million miles away from me, and it was shut down over a decade ago, due to high running costs and maintenance costs etc... Every week on the town's facebook page, there is someone saying 're-open the lido!!!' But it was a lot of hassle and expense for something that was used for around 30-40 days of the year.
I would personally never install an outdoor swimming pool at my property, an indoor one definitely! But not an outdoor one. Not in the UK!
I am bit surprised at the posters saying 8x4 metres 'isn't that big' though, (for a pool in a private garden.) It's 27 ft x 13 ft. hardly tiny! 32ft x 16ft is the average sized private garden outside swimming pool, (in the UK anyway,) so it's not much smaller than that. You could certainly get some decent swimming in.