Hi Anniemaria and welcome to GoY. You don't say where you live in the UK but unless it is pretty much frost free you aren't going to be able to grow this shrubby tree outdoors in winter so it will need to be in a container, filled with ericaceous compost, you can move into a frost free greenhouse. I don't think you can turn it into a standard tree its nature is to be shrubby and the branches to bend downwards. Hopefully someone will have more advice for you.

A. dealbata itself develops a single trunk, but subspecies subalpina is happier with multiple trunks. It can take mild frosts, but there are probably not too many parts of the UK that are warm enough in the winter.