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I think the more expensive ones pack down better - I have a thermarest and it packs down tiny. I also have an Alpkit dozer which, rolled, is about the same size as the bargain 5cm mats from Aldi or Go outdoors. If you have space, not such a big issue. Blanket on the ground is irrelevant with a SIM - you just need your sleeping bag. IME a sleeping bag/pod needs to be at least 3 season not to be cold in the UK - otherwise you might end up needing a blanket over you or wearing more layers.
I don't like thermarests - the ones that are thick enough to accommodate my hips are just enormous! We use exped downmats or synmats which thick enough and warm enough for us to use in January and fold up to a very small pack size.
The ones we use at the moment are a synmat 7 short and a synmat 7 ultra light standard. I started out with a down mat 7, then a downmat 9 for winter and then we used a synmat dlx which we shared, but in the end I needed more space to move or I get back ache.
I have only used thermarests and not any other brands I'm afraid. And all the other outdoorsy people I know use them too. Maybe a decent outdoors shop will be able to give you some advice, or you might find a review online as they do get reviewed and compared by magazines.
Another vote for thermarest here, they're expensive but do have a guarantee, dh just sent a twelve year old one back as it was leaking and they sent him a brand new one. Also the comfiest I've used and pack up really small.