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This is probably a good place to start https://www.lastminutemusicians.com/search/brass_oompah_marching_bands/london.html?sortby=distance
You’ll probably find the price varies hugely dependent on how many musicians you want but a starting idea is probably ~£100 a head for a couple of hours.
Alternatively you could try and charm an amateur local BB outfit if there are any near you. My old one only really did competitions, village fetes and the like but coffers were kept topped up by the odd private gig.
Celebration of course!
No flegs though.