Hi OP. Obviously, Scotland is highly likely to go independent in the near future (and who can blame them, frankly), so the key decision here surely, is what country do you want to live in? I wouldn’t be stressing to much about schools with a 4-month old.
Having said this, I would move south like a shot - solely on the basis of warmer weather / more sunshine hours. That alone brings a different quality of life. Edinburgh is very austere, I find.
Guildford is Guildford. It’s a nice town yes, But quite a suburban feel. And, schools-wise (for a boy), you would be looking at the Royal Grammar which is fine but, I fear, not aspirational enough for your DH. 
If you really want to throw your hat into a tiger-parent-extraordinaire scenario, then please do join us here in SW London. But you will need to get him enrolled NOW into “the right” prep. Otherwise, how will he get into St Paul’s
He will be doomed!
I’m joking, it’s not really that bad (sometimes). But if you want access to the most selective independents, why are you thinking suburbia? Could I suggest Richmond, Barnes, Putney and thereabouts? Or Ealing, Parson’s Green, Fulham etc. Everything on your doorstep. Greenery and parks and the Thames is beautiful here. Some people move to Surrey and generally, they regret it or they just get too bored and come back. Both my neighbours did this and quite a few people I know actually.
Aspirational schools for the DH include - Kings Wimbledon, St Paul’s, Latymer Upper, Westminster, Hampton (super-selectives).
There are other schools which take the 98% that don’t get into the above. But do please bear in mind that even an “average” school will make somewhere like Fettes look like a holiday camp. Be prepared to bring your DS to an entrance exam and queue with 1,400 kids for 140 places in the school. Oh, and he will probably need to sit 5 separate 11 plus exams to get a place somewhere. And maybe sit 7+ or 8+ as well as 11plus.
Still keen.. ? 
No really - why Guildford particularly?