Hi fixmenow,
I think MattressToGo has covered any relevant information about storing your mattress on its side.
Unfortunately I don’t have any specific knowledge about the UK market but there are have been a number of forum members from the UK that have posted here and the information in post #25 here and post #5 here (and the links that they include) should be helpful and provide you with all the basic information, steps, and guidelines that you will need to make the best possible choice.
This would depend on whether your mattress is one sided or two sided.
If your mattress is two sided and has the same design from top to bottom and from bottom to top it can certainly benefit from flipping and rotating on a regular basis (see post #2 here) which would add additional life to the mattress and slow down and even out any foam softening and breakdown on each side of the mattress (see post #3 here).
If your mattress is one sided then the bottom isn’t meant to sleep on and would typically have a relatively thin and firm layer of foam under the innerspring and may use a non skid fabric that is meant to keep the mattress from sliding and would probably be less suitable and comfortable for sleeping on as well. If the top of the mattress is beginning to soften or break down and you are no longer sleeping well on the mattress then in some cases you may be able to extend the useful life of the mattress to some degree (instead of replacing it) by flipping it upside down and adding a softer topper to provide any additional comfort and pressure relief that you may need.
Phoenix