As a Ryeland cross is no different to any other meat breed, I wouldn't pay anything above meat prices. So ask vendors ( if you trust them) roughly what these lambs weigh and offer @ £1.80/kg
If these were born March/April, and depending on whether they're singles or twins, they probably weigh between 20 - 30kg. So I personally wouldn't pay more than £50/animal.
But then I personally would not be selling animals that age at this time, as they'll regress when they're separated from their mothers and there's plenty of grass at the moment to let them grow on some more.