Just for mileage really, not fussed about anything else, cheaper the better, tom tom runner?.....maybe
Garmin 310xt from eBay, you should get the whole bundle including used (urgh) HRM strap for £50 or less. I got one without a HRM and needing a new strap for £20
Tom Tom works for me. Small enough to wear as a watch as well.
Install an app like Strava on your mobile?
Had a tomtom runner, good battery life, nice clear screen. Mounted on a cheap decathlon watch bars mount worked fine.
https://www.decathlon.co.uk/sv-300-handlebar-watch-holder-cycle-computer-accessory-id_3341513.html
Now got a Garmin vivoactive (I'm mainly a runner anyway) which is even better, but not quite as cheap.
Prefer the watch option as it allows me to keep the phone safe and sound hidden away.
OP - I'm about to put a 310xt as mentioned by Jonnyboi above in the classifieds. Used but good condition and includes wrist strap, USB charging cable and I'll also throw in a new basic garmin mount. It's set up as quick release for easy transfer between wrist and handlebars.
Will just be asking for £20 donation to bloodwise through my JustGiving page and I will cover postage. Happy to give your first refusal if you want it.