Doors in the UK are made in both Imperial and Metric sizes. The size you describe is one of the older Imperial sizes, the nearest version being 1981mm x 762mm (78" x 30" - they come in 3" width increments). You will have to trim it down.
The two largest manufacturers are JeldWen and Premdor, both of which do all the range of sizes. You need to be going to a Builders Merchant - not the DIY sheds. Jewson, Travis-Perkins etc - whoever is convenient to you. They will keep the common sizes in stock and be able to order others in about one week. There are three (at least) different weights of moulded panel door - the lightest has a core of paper and 'feels' non-substantial (IMHO); the next has some chipboard in the core and may be your best bet, the heightweight doors are solid chipboard and are fire retardant FD30 rated.
These manufacturers produce a catalogue with SSP in them. However the margins are very large - ask what the best discount is they will offer you for cash sale - builders will be paying about -40% off these SSPs.
Website for JenWeld below in case you want to check out the styles etc.
http://www.jeldwen.co.uk/rangeselected_doors.a sp?range=50