The only option in my humble opinion for a small London garden is to sink the trampoline and add a lid as one of your other replies suggested. We have had this done recently by a brilliant (and local) company called simply sunken trampolines! Their website is
www.sunkentrampolines.co.uk and they did a really great job on our garden. Raised trampolines are so unsightly and take up most of the garden - not to mention the fact that they can blow away in high winds.
We are absolutely delighted with ours and highly recommend the professional company who did the work - they also make and supply the lids which easily take the weight of people walking on top - ours is covered in easy grass which their company sorted and fitted as well.