You have three conflicting goals that only you can weight accordingly:
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.
After about three months of unemployment, today I have been told I will receive two different job offers. Obviously I want to make a decision that maximises my utility, but there are some difficult-to-quantify factors involved. Can anyone suggest a useful or clarifying perspective on the choice? What questions should I be asking myself?
As background, I'm a 36-year old male techie based in the UK. What I would really like to do is build my own startup, or join a startup that I think has good prospects. However, having done that twice, and nearly bankrupted myself twice, I need to get a financial cushion under myself before trying again. For the sake of my CV, I think I need to stay in my next role for at least 18 months. After that, I hope to be able to try something entrepreneurial again, but I want to stay employable in technology as a financial safeguard.
One job is in London, and will certainly pay more. I have many friends there, and a social network I can get back in to straight away. The pool of women I am interested in dating is much bigger there. The job itself will allow me to improve my skills, but probably not broaden them; there are very skilled people there that I can learn from. With this job, I fear being bored, and getting more and more specialised in a skill that is getting less popular.
The other job is in Glasgow. It will pay less, probably by about £15k. The cost of living difference, after tax, will probably be bigger than this - I expect I will have more money in my pocket with this job than with the London job. I only know two people living in Glasgow, so I will need to make new friends and get a new social life. Dating prospects are probably less good, but I don't know by how much. The job itself looks more interesting, and I can broaden my skills. The job market I am in will probably be more open to me after I take this job. With this job, I fear getting depressed due to isolation, 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?
Cost aside, London is a better location. It has more of everything I am interested in (including a LW meetup group!). Location and pay aside, the job in Glasgow is better. I expect it will be more interesting, and will make it less likely that I'll be unemployed in the future.
I don't know how to weight these factors up properly. What mistakes in judgement do I need to avoid? What approaches can I take to make the decision easier to make correctly?