I'd recommend a
Garmin Etrex 20.
AA batteries, buttons, comes with a version of OpenStreetmap maps.
£130, plus £9 for the bike mount.
General points...
Garmins are tough and waterproof - my previous one went bouncing down the road at 20+ mph on several occasions without more than minor scarring, before it died of unrelated causes.
Built-in lithium batteries have a finite life. After two or three years, you'll find the battery doesn't last as long as it used to, and life will continue to get shorter until it's no longer long enough for all your rides. At that point, it will be buy a new GPS - they aren't swappable.
The main advantage of Garmins over other brands (eg Bryton, Magellan) is that they are much commoner, so there are lots of people who can offer advice or help with the supply of different maps.
Route guidance on a GPS will be along the lines of a beep, with on-screen instructions.
Phones can also do voice guidance (depending on what software you are using).
Personally, I'd prefer on-screen guidance - I figure the chances of me missing the start of a voice instruction are too high, and I don't like earbuds on a bike due to the extra wind noise.