> Can he crowdsource this to offset his cost

probably

but the important thing is that you can't spend that much money (well compared to the USA) for a by-election the maximum you can spend is 180k (if youre a member of a national party the party can spend 54K on your behalf. )

in terms of crowdfunding the rules are pretty clear: https://www.electoralcommission.org.uk/guidance-candidates-a...

there is quite a high paper trail bar for doing this.

However if you just want ot get your name on the ballot its £500. if you're just pissing about then thats all it costs.