ScrapCarBuyers.co.uk appears to be working as an
agency for local firms, which would explain why they've got web pages like this one:
http://scrapcarbuyers.co.uk/scrap-car-dealers-required-urgently/
You'd almost certainly get more money by going directly to one of those firms. (Further, I wouldn't deal with any company that fails to include a postal address on its website, as required by UK law, since you wouldn't know where to complain to if you had problems).
Motorwise.com is also an agency. (From their terms of business:"5.1 All payments will be made by the appointed Recycling Company, the details of which will be provided on your quote.
5.2 Payment will be made promptly upon collection of the vehicle and in accordance with the payment terms agreed between the Seller and the Recycling Company. ")
For a reliable local firm, look in Yellow Pages (or on Yell.com
http://www.yell.com ) under 'Car & Vehicle Dismantlers'. Then check out the websites of any likely companies to see if you can find out how long they've been established or (much better) ask at work or down the pub if anyone has used their services.
Alternatively, any firm which is a member of the British Metals Recycling Association is likely to be reliable. (I've just checked the list of member firms around here. They're all long-established, well-known and trustworthy). Enter your postcode here
http://recyclemetals.org/find_a_metal_recycler
and set the 'Specialist Activities' field to 'End of Life Vehicle Authorised Treatment Centre'. (Leave 'All Metals' displayed and the Keywords field blank).