I suppose it doesn't help that I don't understand what a MOSFET is or transistors at all in general.
I read that some solid state relays (which these are, right?)
I read that some solid state relays .. have significant delay
for a fairly hefty £2.64
You don't need an SSR to trigger a flash, you just need an opto isolator. A mosfet changes its resistance from very high to very low, that is what you want to do. A flash gun has a high voltage on the switch contacts maybe 600V or more so it is important to get an opto with an output FET that can handle this sort of voltage. Don't be confused by the isolation voltage it is not the same thing as the breakdown voltage of the output.You can get the AQW280EH from the UK but the postage might be high:-http://uk.farnell.com/panasonic-ew/aqw280eh/photomos-relay-1500v-120ma/dp/1853864?Ntt=AQW280EHmaybe you can get other stuff at the same time or search the site for something similar.Quotefor a fairly hefty £2.64That is a good price for something with that spec.
Is the breakdown voltage the same as the load voltage?
Farnell, but their prices are pretty much the same as Mouser's