I never found a "weekend trip" from UK a particularly efficient way of travelling to WHS, either in terms of money or in use of time.
I normally do extended weekend trips, i.e. I add Monday or Friday. For me right now, time (vacation days) is the constraint, so saving my holidays for the one big vacation I do each year is really a priority. I also have a quirk in my contract where I get individual extra days off for business travels, so doing a 3 day vacation doesn't really count as holiday. With Hamburg airport rather close to the city center (20min or so), I can leave work at 3 p.m. on Thursday afternoon and fly out as 5 p.m. (2h). Add 1h flight plus 1h commute and it's 4h, not too much.
On top, with all the cheap flight options available in Europe nowadays (Easyjet, Ryanair, ...) and being smart about booking flights (flexible schedule/destination, using google flights, combining with work trips) the flights aren't that much more expensive than a weekend trip by train to see my brother in Leipzig. Only for longer trips am I willing to pay more than 150 back and forth.
I place it at around Heilbronn!!
I would place it in London Gatwick from where loads of Easyjet flights connect you to all over Europe.
every site in the continental US and Canada
Not that great a feat... Germany has the same amount of sites as those two combined ;)
Closest would be Canterbury (506km) and Holasovice (766km).
Holasovice, Clyde, get in your car now and drive there. It's the best! LOL. Honestly, makes only sense to combine with Cesky Krumlov which is great. Holasovice is really underwhelming.