I went through this dilema recently (also in the UK, there aren’t many)
I ended up only getting 10kg or so, but its very top heavy and feels surprisingly heavy!
I went with the Xn8 Sports Vest (from a well known online marketplace) . Its not adjustable for weight, but very comfortable compared to some of the ‘wearable bags of weights’ you get, which does help with using ir more often. Can easily wear it under a jacket or around the house.