Sounds like a very high offer from Webuyanycar for an older vehicle. I would suggest that unless your car is in pristine showroom condition, that figure will reduce drastically once they've marked down every single mark on the car.
Not many people know this, but Webuyanycar stock all gets sold through BCA car auctions so prices are never going to be great unless it's newer stock that they know has a high demand.
As far as the bumper repair goes, if the car is going anyway, sell private as is with a bit of a reduction (Autotrader priced £100 less than other Jazz under 100k within 50miles). You'll likely not recover any money spent replacing bumper, a home spray repair job may make buyers suspicious (as it will be nigh on impossible to hide) & the ice blue paint seems to be notoriously crap on these anyway.