I know it FEELS like a 500£ phone, but if you look at ebay it's more like a 35-40£ phone today. Perhaps this helps reframe it? If Google is offering to give you back your original 500£, that seems like an awfully good deal.

I, personally, just went through a similar situation, except my 6p was out of warranty. I use Project Fi and have the device protection $5/mo add-on, and they sent me a top of the line Nexus XL to replace my top of the line 6p, for $100 (the device replacement cost). My 6p also got stuck in the boot loop.

Perhaps ask if a Nexus XL (not Nexus 2) is a replacement option?

Honestly, my 6p was damn near unusable before the replacement. I wouldn't say I have a lot of apps installed, I basically install a set of the ones I use daily and rarely install new ones, but it was incredibly laggy. The Nexus is way better in that regard. Yes, I had some apps I could remove, but pretty much everything on the phone was something I used regularly, most I used daily.

Aside: I was looking at a recent sale on the Nexus 2 XL, which brought it down to more like $800, and they offered $180 trade in on my Nexus XL. I felt like that was kind of low, but when I looked at ebay it turned out that was about right. I decided I'm super happy with my $180 phone! :-)