The UK, especially London, is expensive. Budget travelers need £70-90/day in London: accommodation alone in a hostel dorm costs £30-50. However, London's major museums are free, which helps enormously. The Oyster card or contactless payment gives access to the Tube and buses at capped daily rates.
Outside London, costs drop significantly. A B&B in the Cotswolds or Scottish Highlands runs £60-90/night. Mid-range travelers (£130-180/day in London) can stay in comfortable hotels and eat at good gastropubs. National Express buses are far cheaper than trains for intercity travel.