I think the points that @hannahcolobus and others are made are very valid about all the extra expenses. But I think you’ll only get an idea of whether his charges are competitive if you get a couple of other quotes.
Don’t bother getting a British Gas quote - as PP said, their prices are extortionate. The guy who came out from BG to us was also a real nob, and very sniffy about the quality of any non-BG engineers.
Our heating engineer charges in the region of £350 per day for his labour. We’re doing full-scale renovations at the moment and I know the day rates for my heating engineer/plumber/carpenter/electrician/plasterer. The trades I have found are all really transparent over costs. They’re not necessarily the cheapest in the market but they’re competitive, good quality and very importantly, not misleading over cost.
OP, you say that the engineer has said it’s only one day of work. If that’s the case, I think your quote is a bit steep, even considering the other factors. We had a new boiler relocated to a completely different part of the house, plus loads of work to sort out crappy pipework. That took a full week of work but it only cost £600 more than you’ve been quoted.
I would get comparative quotes. Mind you, I’d also pick the quote from the engineer who is transparent, professional, and communicates clearly, I wouldn’t automatically plump for the cheapest one. I’ve learnt from bitter experience!