Danni, You will often find its more economic to buy standard lengths than cut too size, most suppliers have a cutting charge anywhere from around £2/3 & more per cut !
Gauge plate is sold in 18″ lengths — from the year dot ! as SS is 13″
If you have to buy more of a material mark the surplus with a colour code so you know what it is for future use — you will use it over time, material cost is usually the lowest cost of anything you make.
Look around Manchester there will certainly be steel suppliers or users you can maybe collect from ?
John