If you are an odd measurement they add 5". Plus, they don't measure firmly enough anyway. I'm a fairly definite 30" measurement, they measured me too loosely so ended up with 31", then added 5" so came up with a 36 back...I could have tied it in a bow!
In that case I would be pretty surprised if you are a 38. I'm roughly a 14 on top (although I am busty) and wear a 30. Most 14s tend to be somewhere around a 32-34 (it's not an exact relationship but can give a rough indication)
I was measured in m & s as a 36A many years ago, but found through trial and error that a 34B or C worked better. And then about a month ago I read one of the bra intervention threads and now I'm wearing 32DD or E depending on brand!
However I recently bought one the wonderbras in my link above to match my "new" size but it just didn't work so well in that style. The trouble was that most DD cup bras are styled for large breasts and lots of cover, and it just didn't work well on me with that particular bra. The bra guru's "scooping" method works wonderfully on all my other bras, but for some reason not on that one (perhaps due to skimpiness) so I have stuck to my old 34C.
In that case chances are you aren't a "true" 38B. To give you an idea, Sorrel and I visited M&S (as did some other ladies) to test out their bra fitting. We both know our bra fitting stuff - I get fitted at Bravissimo myself.
Me - true size is around a 30HH. M&S tried to put me in a 36F Sorrel - true size around 30G/GG. M&S - tried to say a 36 or 38D.
There is some information about how a bra should fit here
crushedintherush Before trying to suggest some suitable bras, can I ask whether you are certain you're a 38B? It's just that if M&S if your usual place for bras and you've been measured/fitted by them, then in all probability, your current size will be wrong.
The picture looks a bit crap, but I find it really useful, can be worn with absolutely any style of top, and in fact I mostly wear it strapless. I also think the cups come up a bit small, so you might want to size up.