Why do you need a discount ? You have obviously done some research to be so specific on which make and model you want, why not just pay the going rate if that is the one you want.
Or turn up on the sellers doorstep holding cash or buy it using a card where you get points, air miles or cashback.
Really speaking I doubt there is much leverage to be had on something that is under £130. One more point .. it is the wrong time of the year to be hoping for discounts on lawnmowers. Very shortly they will be running out of stock when everyone wants a new one with spring almost on us !
Possibly the best price in the UK at the moment .. but even they are out of stock.
https://www.keengardener.co.uk/cobra-gtrm43-17-electric-lawn-mower.html?utm_source=TradeTracker&utm_medium=16740