Lidl are offering a petrol engined rotary lawnmower from the 31st of March 2014 priced at £199. It is branded Florabest which I believe is a Lidl own brand.
It is fitted with a Briggs & Stratton 575 EX series 140cc 4 stroke petrol engine.
Other features include a 46cm (18″) cutting width, 7 cutting heights, a mulching attachment and a 60 litre grass collector bag.
2016 UPDATE : Lidl are selling a slightly smaller petrol mower for 2016, details here.
UPDATE: Homebase are offering a self propelled 46cm Qualcast lawnmower with a Briggs & Stratton engine for £199.99.
UPDATE: Yet another contender could be the slightly more expensive Einhell GH-PM 46/1 S.
UPDATE: Another slightly more expensive self propelled 46cm mower is the Oleo-Mac G48-TK.