This might be a bit specialist but none the less a great price for this UPS
Next cheapest, via google shopping is £393.60
Liebert GXT3 is a full feature UPS providing reliable protection for IT network equipment and data centers. It is a true on-line UPS that delivers continuous, high-quality AC power to IT equipment with no break when transferring to battery. It protects equipment from virtually all power disturbances due to blackouts, brownouts, sags, surges or noise interference.