Quick answer: London runs about EUR 170 per day mid-range - noticeably less if you eat local and walk. Cheapest window: May-June and September - mild weather, long daylight and parks at their best; pack a rain layer regardless.
What things actually cost
flat white
4 EUR
pub pint
7 EUR
tube fare in Zone 1 (contactless)
3.20 EUR
fish and chips
15 EUR
Tower of London entry
39 EUR
major museums
free
Eat well for little
Fish and chips at a proper chippy - 14-17 EUR, ask for it with mushy peas
Borough Market street food - grilled cheese toasties or salt beef bagels, 8-12 EUR
Sunday roast at a pub - 20-25 EUR, book a table, they sell out by 2pm
Curry on Brick Lane or in Tooting - around 15 EUR for a main and naan
Beigel Bake on Brick Lane - salt beef beigel for about 7 EUR, open 24 hours
Money mistakes to avoid
Paying for attractions when the biggest museums (British Museum, Tate Modern, Natural History) are completely free
Buying paper travel tickets - just tap a contactless card on the tube, fares cap automatically at about 10 EUR a day
Underestimating distances - 'central London' spans miles, so group each day's sights by area instead of criss-crossing
Booking a hotel far out in Zone 4+ to save money - the commute cost and time usually cancel out the savings
Transport without the tourist tax
From Heathrow, the Elizabeth Line reaches central London in about 35 minutes for roughly 15 EUR. In town, tap contactless on the tube and buses; fares cap around 10 EUR per day, and central sights cluster close enough to walk between many of them.