It is all too easy to damage a number plate on your vehicle, they can suffer from other careless drivers in car parks and are often the first thing to strike a high kerb or low wall in the dark. Furthermore, if the cheaper types have been badly fitted by a previous owner (as mine were), the backing can deteriorate and they will require replacement.
Since 2001, you can no longer make up your own (legal) number plates as you once used to be able to, by buying the required self-adhesive letters and other parts and setting it out yourself.
Now you have to get road legal number plates made up by a DVLA registered supplier, who will add their name, a short form of address and their postcode to the plate. this is to reduce the incidence of vehicle "cloning" by putting someone elses plates on the vehicle being driven, with the legal owner then getting any resulting flak.
For older vehicles, which may only have a short life before being sold on or scrapped, the cheaper plates are the obvious choice. You can expect to pay from £7.00 per pair (including delivery) upwards from eBay vendors. This compares very well to the quote from my usual local supplier of £10.00 each, i.e £20.00 per pair - 3 times the price!
In specifying your number plates, remember that the minimum legal height for the single line rectangular plate is 101mm, but the plate itself is likely to be 115mm height, which is the more typical dimension on stock plate blanks. This may mean that your number plate protrudes above/below your bumper and can still be caught on obstructions or suffer deterioration.
The minimum width is determined by the make up of the letters, numbers and spaces in your registration number. Most stock plate blanks will be large enough to accommodate the maximum length registration mark and also a Euro/GB symbol.