My advice is to try Australia or Canada, if you are coming without a partner, at least you can apply for PR or work permit after your studies, UK not any more, if you finish your studies-go home
According to where I live these are the cost normally
Average cost for a month- rent will be £250-£400
Food-depends how you r going to do, if you going to cook very cheep
Internet- most of the room rents include internet
Mobile charge - from £10 you can get a latest phone with a contract for between £30-£40 per month, but to get that you need a good credit score, which will need you to have reasonable monthly income and you need to maintain your bank account- which will take some time
or else some network offers you to make a refundable deposit like £100 to get the contract, they will pay it back in 2 months or so, but still you need to prove your income.
Cheapest transport- with in London oyster card is the cheapest way get a weekly or monthly card- you can use any mode of transport
other areas, i think you need to get travel card separately for trains and buses.
Cheapest way to receive money from Sri Lanka- this I don't know/ I got a BOC account here so can deposit from SL
part time job- This is the main problem, you will only have 20hrs per week, cash in hand jobs are difficult to find these days.
They have lot of vacancy in care homes, if you are up to that kind of a job, but only 20hrs, coz they don't usually do cash in hand.