However, both of the above formats are unavailable to users in the UK.
The iPad Marvel app seems to work just fine for thousands of comics. If you want to buy.
If not, then the usual sources as mentioned her already provided huge amounts of comic, from very old the bang up to date, which can be viewed using one of the fine freebie viewers in CBR or CBZ format.
All you have to do is your homework. Seek, and ye shall fine.
BTW, all of them look fabulous on a Retina screen.