I disagree with lots of the PP here.
Let's say it was a rising market - if your sellers had asked to renegotiate, and said that because prices had gone up since your initial offer and they now wanted an extra £33k just before exchange, what would you think?
Would you think they were reasonable asking for more money just as you're about to exchange contracts? Or would you feel that they were being greedy and that they shouldn't get any more because you had an agreement in place?
If you would willingly pay your sellers more in a rising market without complaint, then by all means go ahead and renegotiate downwards.
But if you would feel mightily pissed off at being asked for more money at the last minute, then I think it's incredibly shitty to try and lower the price because the market has dropped.
You can't have it both ways - you should hold yourself to the same standards that you'd expect of your seller.
There have been occasional posts on here when the market was hot about sellers asking for more money very late on in the process due to increases in valuation - and almost unanimously posters have said that it's cheeky and unfair. So I really don't understand why it's OK in reverse.
I'm also not sure why you were continuing to look at houses - once you're committed to buying, it's a bad idea as the chances are you'll see something else and start doubting your decision! If you are happy with the offer you made, stop looking around. It will only unsettle you - as proven here.