I had a similar problem many years ago- it turned out that the culprit was biological washing powder. There are enzymes in the powder to make whites shine, the problem is that many people are alergic to them.
In my case I changed biological powder for soft soap powder, washed everything 3 times and the itchy rash disappeared.
You will have to provide proof of where you will be staying and for how long in each place.
Me and my wife went to UK for a month in 16 for the wedding of my daughter, we stayed with a friend in London, then a hotel in Norwich and then on to Scotland.
I got a letter and passport copy from the 2 friends I stayed with and included a copy of the hotel reservation and a copy of the wedding invitation.
Be familiar with the details on the visa application. When we arrived at Heathrow I went though the non Brit channel with my wife. The IO was very friendly and chatty, but with a purpose. He was asking in a conversational way about info we have on the application. We hadn't BS about anything, so it wasn't a problem. I presume he was looking to see how I answered his questions
I am applying in Kanchanaburi and I had to have my passport certified by the British embassy in Bangkok. You get the forms online at the embassy web page and there are links on there to make payment. You can't do it in person at the embassy Andy more.
When I was there I needed local currency for the hotel, the taxi driver knew exactly where to go and actually got us a good exchange rate. Give it a try, they know all the best places