Pricing using overseas costs is rather silly, however if somebody is looking to buy something a bit more niche like what you have… the probability of those people will know it’s cheaper to import and would opt to do so, unfortunately.
From ProAct, including shipping & import fees it’s roughly £672. Meanwhile HPA-UK have it up at £840.
The extra mag is £45 from proact with their shipping. Again, people that are into GBBR’s mostly know that mags are cheaper overseas and will order from there rather than in the UK most of the time. HPA-UK have the mag at £65
I don’t know how desirable the LPVO would be as it’s not one of the popular brands (nuprol optics are kinda naff on gbbrs in my experience too as nearly all of the ones I’ve tried have got damaged with use on VFC AR’s…) but it looks to retail for £100
So with UK prices, retail would be £1005, 65% would be £653.25
Using overseas prices, retail would be £817, 65% of that would be £531.05
So your price is pretty high. I’d go in the middle of those two prices above at £635 personally.
I also don’t think you can describe it “as new” when it’s had 15 mags put through it and an accessory mounted. It’s used, because it has been, well… used