silicone paper is baking paper and Sainsbury's will definitely have it. In fact - here it is
It's not hard to wrap the cake, just a bit fiddly.
Best bet is to grease the tin with butter first.
Roll out a length of paper and place your tin on it. Use a pencil to draw around the tin and hey presto, you have a guide for cutting out the circle for the bottom of the tin. Then cut long, thin strips of paper tall enough to go from the bottom of the tin to about 10cms above the top rim. (so if your tin is 8cm's high, your paper should be 18cms high)
Place these in your tin - This is where the greasing helps as the paper will stick to the butter and stay in place more easily. The cake mix will hold it in place once it's in too.
You can do the straight side pieces in 2 or more sections if it makes it easier to cut just so long as the sides are covered all the way round.
For the top. Cut a piece of paper a bit more than twice the length of the width of your tin. Fold it in half. You now need to cut a 50p sized hole out of the center - I do this by folding it in half again and cutting a semi circle out of the center of the edge I've just folded.
I then fold each of the 4 corners down to help stop the paper blowing off the top of the cake while it's in my fan oven. I end up with a rough octogon shape.
It's not hard, it's just a bit of a faff. The paper just stops the cake sticking to the tin, it's not an absolute essential.
If you haven't got one already, I would recommend a spring form cake tin - the kind where the bottom comes out (sainsbury's have these too - here) as it makes getting the cake out soooo much easier.
Does that help?