Theoretically, 'non-tailored outlets' use the same measuring system as Saville Row tailors, so you should be looking to purchase a jacket with a 46" chest. However, no two companies seem to use the same sizing system. It's a bit like shoes. I sometimes need to buy size 8, more often size 9 and occasionally size 10. Every manufacturer seems to be different.
If you go to a High Street shop, start by trying a jacket with a 46" chest. It might well fit perfectly. However, when you try a different design (from a different manufacturer) you might find that a (nominal) 44" or 48" chest fits better.
If you're thinking of buying online, a 46" chest will probably be a 'fairly good' fit but it might not be 'perfect'.
Chris
(PS: Please ignore Chino-cube, alias A-New-User alias Lovablemrkno alias Noknowledge alias god knows what! He seems to be looking down his nose at me because I recently delivered the local Yellow Pages, despite the fact that I was getting �12 an hour for a fairly pleasant walk or, taking into account the administrative duties I also did, about �20 an hour in total).