I just can't see the UBW relationship working out in the long term.
Sure Rin and Shirou both got into the clock tower, but Shirou sucks at doing normal magic and if he whips out Unlimited Blade Works then the wizarding equivalent of the CIA is going to go down hard on his ass. Even if he could somehow play catchup I just don't see him being able to keep up and staying in London is just going to mean giving up his dream of being a Hero of Justice.
And it turns out Ufotable thought the same thing since at the very end he ditched Rin to go fight terrorists in the middle east. He won't turn into Archer but long distance relationships are a lot harder than it looks to keep up, especially since Shirou will constantly be in danger during his hero duties. There's also basically no chance of Rin abandoning her duties to go fight ISIS alongside him so there's also that. It could work, but I'm just not convinced it's going to.
But then again for saber Shirou only knew her for like 2 weeks before she left, he then spent the rest of his life looking for her under he finally finds her in the afterlife so maintaining a long distance relationship doesn't seem anywhere near as farfetched compared to that.
HF isn't as flashy as the others but I can't see them doing anything but stay together for the rest of their lives living in peace.