Hi, here in OZ you can buy small prepacked rolls of solder, for normal, silver and aluminium, there is only about 300mm in each roll,The UK probably has the same products in the handyman section of your DIY store.
The last time I welded aluminium, was when I built a forward cab on my 17 foot fishing boat, even though I am a welder, aluminium is not my thing, tying to stop the metal over heating using oxy was a real hassle, cause you have to put in a lot of heat for marine grade Al.
Good luck with the low temp soldering, Lumiweld can some times also be a hassle
John Holloway.