I looked at the actual cameras, I already own a Xiaomi phone, and it's excellent - 1/3 of the price of Samsung.
Anyway, their cameras seem to be a good way to go as they are a bit of a 'brand' as far as Chinese companies go, and they use the same mount as Go Pro so the accessories should all be cross-compatible
Yi £39
https://www.amazon.co.uk/dp/B016MAUHOKYi Lite £65
https://www.amazon.co.uk/dp/B0742B56ND/ Same camera performance as above but with a screen, EIS, bundled waterpoof case (£7.99 otherwise). Bicycle mount is £5.60 with silly voucher
Yi 4K £120
https://www.amazon.co.uk/dp/B01I40DE6Y/ Better camera sensor + 4k. EIS at 1080p but not 4k. Waterproof case bundle + free handlebar mount is extra £5
Yi 4k+ Same as above but with EIS at 4k thanks upgraded CPU apparently sells for around £207 sometimes but currently more than that.
Their Gimbal is ~£75 depending on which silly voucher you get.
Various reviews suggest that the Yis' EIS compares poorly to GoPro, etc., which isn't such a surprise really. The 4k+ seems to be completely pointless therefore.
The current best selling brand on Amazon - Akaso - is more expensive & inferior, just goes to show how effective review manipulation is.
Xiaomi have cut their prices on the Yi 4k quite a bit since launch (as they tend to do) so it seems to be fairly good value atm - £125 compared with £70.60 for the Yi Lite with bike mount
edit: apparently bike mounts are more-or-less useless unless your surface is super-smooth - you want a chest mount.