After reserving myself a Garmin Nuvi 710 as below.http://www.hotukdeals.com/item/306739/garmin-nuvi-710-sat-nav-with-traffi/
I was able to reserve one at my local store in Croydon. When I went to pick it up a couple of hours later the guy at the desk said they didn't have any in stock and offer me a Garmin 255 TW (meaning Widescreen + Lifetime live traffic updates) for only £109.99. I said I would give him £99.99. he said he had to go and check, after a few minutes he returned and agreed I could have it for that price.
I know it may be store specific but maybe some other could find this usefull.
N.B. Very important that you take a printout of the reservation with you or they will just say they don't have any.