I used to use a Garmin eTrex Vista HCx for navigating on the bike, no OS maps on it but was good enough for a breadcrumb trail to follow. Then the USB socket failed on it after 4 years of abuse, so couldn't load routes onto it anymore.
I decided to upgrade to something with OS maps, spotted the PSA regarding the halfords deal for the edge 800 on here but by the time I'd got round to it they'd sold out.
Glad they did to be honest, I actually ordered a garmin etrex 20, got it for under £140 by using go outdoors' price beat against egalaxy.co.uk. Handlebar mount was then £9 again using go outdoors' price beat against amazon.co.uk.
So, all in £150 investment. Then just needed maps, you can get full UK OS coverage for free from various places (there's another thread on here at the moment discussing sources for maps for the edge 800 that can be applied to the etrex 20 too).
The unit itself is a nice size, screen is big enough to follow route on and the whole thing feels very very robust as its designed for outdoor activities not just mounting on a bike. Also it uses proper buttons not a touch screen so you can use it with gloves on.
So, overall I'm pretty pleased, I saved a fortune and have full OS mapping on the bike. Kind of glad I missed out now!