HenriettaPie Bravissimo are the best on the high street (they don't actually measure you, they fit by eye) followed by some branches of Debenhams, JL and HoF. Practically everywhere else (many independents excluded) continues to add inches.
RevealTheHiddenBeach Yes, loosely, no bra. This is important as you're ensuring you are including 'all' breast tissue. Bear in mind that the cup size is just a starting point. I need a bigger cup than I measure for example. Some people need a smaller one.
PsychicPaper OK, the central gore not lying flat indicates that the cups are too small (and the style is possibly wrong too). Your measurements indicate a starting point of 48DD which is much smaller cup size (in volume) than you're currently wearing. A 48DD is equivalent to a 46E and a 44F. Are you sure those measurements (48/53) are correct?
The fact that you're already wearing a 44 indicates that you are able to wear a smaller band than you measure. Many of us with more squish on our torsos can do this (and need to for support!). Added to which is the fact that the bigger the band, the more elastic and thus more stretch in % terms.
Wearing your 44H/HH, can you scoop and tell me if there is overspill/bulge or gaping? If you put it on back to front, how far can you pull the band away from your body?
Panache are very stiff bands and it may be that you need to try a different make (e.g. Elomi or Fantasie) which is a bit softer.