The excellent SteelSeries Rival 3 Wireless is down to £25 at Amazon UK | Rock Paper Shotgun
SteelSeries Rival 3 Wireless - Wireless Gaming Mouse - 400+ Hour Battery Life - Dual Wireless 2.4 GHz and Bluetooth 5.0 - 60 Million Clicks - 18,000 CPI , Black : Amazon.co.uk: PC & Video Games