Oh dear, we may have to rethink this one.
Thanks so much for the very helpful comments, I'll answer some questions.
"What is the roof of the kitchen and family room made out of - it looks like plastic?"
It's a bit worse than this, I did some digging. According to the planning portal this extension is a 'conservatory' so no building regs. Roof made of PVC and walls of hardwood boards. Seems unlikely this can be used as a kitchen legally (no subsequent approval for conversion).
"I imagine they say needs an outdoor grade door between them and the house."
Yeah agreed.
Loving the Swingers comments 
"wouldn't of thought it would cost a fortune as you could reuse the existing kitchen units"
This was my hope, it's so bizarre that maybe we could get it reduced and turn it into something normal, but with the kitchen being a conservatory the price is way too high.
"Am I reading the floor plan right—is the galley kitchen the main entrance?!? Is that even safe on fire safety grounds?"
Given that it's a conservatory....
"I get the impression from your OP that the house is near the top of your budget. This place is going to be a money pit, imo."
Yes you are correct, we could reach to 440 or so but that would leave nothing left for upgrades and so on. Plan was to get this for maybe 410/420 and then upgrade it over the years (as it is liveable). But not without building regs...
We'll go see it next week.
Since you've all been so helpful would you mind commenting on this as well?
www.rightmove.co.uk/properties/78715119#/
it's a bit small for us but we'd hope to extend the kitchen into a kitchen/diner and maybe extend the rear of the garage to make a bedroom 4. only sad part is the small bathrooms but I don't think that can be fixed.
Thanks for all the help!