I take some cash but never been asked for proof of funds, however better safe than sorry as the decision whether you enter the Kingdom, or not is up to an individual
You will need to look at and follow the UK immigration site. Arduous but it lets you fill in all the required info including uploading the required documents. I did it last year for my girlfriend and we came to UK for 2 months. Get all your details listed and prepare the two files you have plenty of time and good luck
Try opening a bank account back home , UK in my case. Then try to take out £5k in cash, you will be asked all sorts of questions so B4 you criticise look at it carefully they are trying to combat fraud.