We only ever stay in a Travelodge in London. If you book now at full price, then look out for a cheap rate nearer the time you can cancel the full rate booking. Some are always expensive, some are often a lot cheaper.
I would not stay at the Travelodge Kings cross or kings cross Scott both are truly awful and as such I will not set foot in Travelodge again.
The Kings cross one messed up early booking so couldn't leave luggage or go anywhere rooms were dirty
Pull out bed had 15 - 20 nails sticking up we only discovered this when my daughter stood on the bed and got one on her foot :( an tbh the staff couldn't give a monkeys . They short measure in the bar , the tables had pieces coming if the side which cut my other daughters hand when mentioned the table was sellotaped together !!!never again
Kings cross Scott I was quite poorly and very cold so asked for another duvet no none spare . The curtai in the room didn't cover the window so I asked if they could hang a sheet for me to cover the gap ( am not fussy ) to block out the street light that threw its beam right across my pillow . Nope no can do . There were 7 teaspoons down the plug hole and I did not want to take me shoes off as the carpet was vile . Nasty nasty nasty .
Try Premier Inn, have found these to be fine. Much better than Travelodge. Although as it is marathon weekend. You may not get the deals that weekend as they know they can fill the rooms.
As its a premium weekend in London you're likely to pay top prices at hotels. Why not try the bed and breakfasts / guest houses? Travelodge are OK but not where I would chose at all, same for Ibis Budget. Premier Inn though - very good but probably pricey at that time.