If that is the market average, ie what you can find the same bike in similar condition selling for, then fair enough. If that's just what their stupid Glass's guide says, tell them where to stick it and refuse the offer!
Throw it back, with as many examples of actual sales as you can find - NOTE: These must be outright sales, not auctions. Autotrader ads are fine. Buy It Now eBay sales are fine.
normally its 10% to buy the bike back at the market value so £200.
Cost of buying the salvage back is usually 10%, up to a certain value. Next it'll be 15% up to another value and finally 30% if it's any higher.
Ideally, don't let them take the bike away, especially if it's the Third Party insurers who are organising the valuation.
One trick is to take away a bike that would cost £500 to repair, break a few bits and then claim it would cost £3000. This means they can write it off and go so far as to 'discover' pre-accident defects that then affect their pre-accident valuation.
Remember also that their estimate of repair will include fixing every single scratch and dent, all with OEM parts. If you have any fairing, you're automatically up the creek on that front!