You have three conflicting goals that only you can weight accordingly:

• Work at a successful startup (fame, fortune, culture or vision)
• Find a mate / companionship
• Employment market desirability hedge (what-if...)

What I would really like to do is build my own startup, or join a startup that I think has good prospects.

Winner: London.

The pool of women I am interested in dating is much bigger there. Dating prospects are probably less good [...]

Winner: London.

[...] and also the long-term effect of accepting a pay cut - if I worked in London again later, would I be able to negotiate my salary back up?

Winner: London.

Everything else sounds like noise, or sounds like they should be assigned lower weightings than the three concerns above. Another factor that you only can answer is how much extra time you'll carve out to keep up desirable startup skills (designer? developer? business?) while in London, as you mentioned that the London job will only allow you to specialize in an unattractive skill. This statement alone makes me wonder exactly what your previous two startup roles were -- if your skillset was good enough to join two prior startups, why would 18months of another skill diminish your existing desirability, both to startups and larger organizations in general?

Until you can provide weightings on the above, London sounds like the better choice due to the flexibility it offers you in the future, provided you have enough motivation and discipline to (a) stay actively engaged in the (presumably) larger London startup/tech scene; (b) spend (some) spare time on side projects to remain desirable to both future large employers and startups; (c) find a mate.

One counterpoint that you already mentioned was that Glasgow would give you more money after-tax. If the Glasgow vs. London after-tax income delta is large, and if your true primary 18mo goal is to "[...] get a financial cushion under myself before trying again", then clearly Glasgow is the way to go. Also, quick Google searches show that there seems to be more interest in boosting the Scottish tech scene in Glasgow and Edinburough. Depends on how serious you are about jumping back into the startup scene after 18 months.