any Decent auto electrical shop can do it. There are a few around you that have good feed back. Dyfed auto electrical for one. Never used them. You have shorts in Swansea.
I bought an electrical hook up from go outdoors I lob it out of a small gap in my pop top canvas.
You can buy the kit to do a leisure battery from Jk go outdoors sell the battery itself never done it but have a go or get it all in for a travelling auto electric guy to fit. Again there are a few according to google around you.