Baggage delayed two days. How should I proceed?

I would like to know what do you recommend to get my compensation from united airlines, as my mum came to see me in England, and her suitcase arrived 2 days later. In the airport they told us that she can spend some money buying toiletries, some clothing like underwear and those sort of things, while she gets her suitcase back. So we went to buy some of those things, and we spent about £120, which I think is fine, and not a lot of money.
On united website is everything so unclear about the claiming, there is not an email address, there is just a fax number and an USA address, so we filled the form with the receipts, copy of passport and tickets, everything, and the fax number doesn't work, so we posted the letter under recorded delivery. After 2 weeks, we tried to contact them because we haven't received anything from them, not even an email saying that they've have got our letter, and united said sorry but we don't have any letters can you spend some more money to post it again!! They don't have a telephone number to complaint in uk. and also, they rent a company which is in charge of the suitcases, once my mum didn't get her suitcase united asked you to get in contact with the baggage company, as they won't know anything about your suitcase, so you don't know these things until you don't get your suitcase with you in the airport.
Any suggestions? I just want my compensation!
thank you