I do think that we, the commentariat of Omega Forums, should develop a formula or theory for valuing such watches. It's really tricky.
On the one hand, it's not a piece that a hardcore collector would want. You say you're after a 1960s Seamaster, and I'm guessing that means you'd like a collectible watch that might hold value and be an heirloom piece that Omegaphiles will always deem worthy of a respectable collection. Whatever it says in the ad, this isn't that watch. Once the dial has been adulterated--no matter how attractively--it's lost its originality, and that tends to the be the highest standard for serious collectors. (There are even some among us who would prefer a badly scratched case that's never been polished to one that's been polished and looks brand new.)
On the other hand, it's still an Omega at its core, with the right movement inside, presumably working properly. And it's a good-looking watch, not one of those shoddily-re-dialed overpolished atrocities that are so common on eBay and Chrono24. I'm not a turquoise watch guy, but if I were, I'd wear that very happily. And 99% of the 1% of people who'd even notice my watch and see that it's an Omega wouldn't have any inkling that it didn't come from the factory looking just like that. (If such things even matter.)
I think valuing the watch comes down to a basic question: to what
extent is this still an Omega? £1,450 is an Omega price---and a high-side one, seeing as 14763s sell from $500 USD up to about 2k, and that's for watches that haven't been modded.
Related to this, there's the twisty logic of how much the modification hurts the watch's Omega-value; likewise, if you think of this as a pretty blue watch, how much does the Omega-factor increase the value of what might otherwise be
just a pretty blue watch?
---TL;DR---
Just my opinion: this is not a watch I'd still consider to be an Omega in any collectible sense, so I would not want to pay an Omega price for it---it's gone from being generally collectible to being a niche fashion piece, and I think the mod, as good as it looks, only hurts its monetary value (unlike some of the high-end, brand-name Bamford mods and the like that people do). I'd choose to see it now as a pretty blue watch that gains value from the vestigial Omega hardware and branding.
Fossil makes something superficially similar for about $300. In my estimation, I'd say the Omega hardware and branding adds about $500 in physical and emotional value. $800 (about £620) is probably fair.