I started out on the PS3 version from launch and play on the PS4 now, using a fibre line in That London. I had some occasional issues with lag and dropouts when playing over Wi-Fi, but since I switched to using a wired connection on my PS4, I've never had any trouble that wasn't the servers' fault. If it helps, my ISP is EE.
The GAF FC has a decent core of EU players. As for the general Ultros population, depends what you mean during EU hours really. During the day, it is pretty quiet, no way around it. It starts to pick up in the evening, but if you want to raid, you're probably going to have to go out of your way to find a group of people who run reasonable UK times. But since you're new to the game, maybe raiding is a bit of a secondary concern for the time being. Although progress through the story and the journey from level 1 - 50 is gated by dungeons and bosses, a great deal of it is done solo, so for that part of the game at least, I don't think playing on an NA server is such a big deal. I'd estimate that playing through the main campaign and going from 1-50 takes about three weeks with casual play, so there's a lot to do. There's also a lot of extra stuff you can do solo like crafting items or gathering items for crafts depending on what you want to get out of the game. (I like to craft, gather and create materia to sell when the server is quiet. It's pretty nice to spread stuff around the market boards and then see notifications of sales start to roll in when the NA players get on.)
You might want to consider rolling a tank (gladiator or marauder) as your first class - that way even during the slow periods before most of the North Americans wake up, you can still easily get through queues for dungeons/bosses. The downside is that tanking is a lot more responsibility and you might need thick skin.
Anyway, I hope the above doesn't sound too discouraging about playing on Ultros from the UK, I just wanted to try and paint a fair picture so you could draw your own conclusions. I'm personally fine with it, and activity in the GAF FC is more EU-friendly than I expected, so suck it and see, I guess. Once you've done the initial bit of story to open the game up (levels 1-15), you can get through it pretty quickly a second time (since you'll know exactly what to do and you can skip a lot of cutscenes without missing anything), so if you don't like it on Ultros, it's no big deal to reroll. Or you could drop 12 squid on a world transfer if you're really attached.