I'd also advise you to go to Bravissimo anyway. I've never had anything other than exemplary service there, and it's still my bra-buying destination of choice despite being a two-hour drive away from home.
Avoid Mothercare at all costs, though! I've been to two branches of Mothercare in the last week to try and buy a bra, as I won't be able to get to Bravissimo for another few weeks and am bursting out of the bras I'm currently wearing.
I was a 32FF pre-preg and, at 28 weeks, am now in a 34G, which has become too small in the last week alone.
A couple of days ago, a really rather rude and dismissive sales assistant in Mothercare, who clearly wanted to be back on the shop floor rather than bothering with me, measured me as a 38D. I knew that was wrong, but she insisted I try it anyway. Quelle surprise, it was huge around the back and far too small on the cup. It gave me no support at all, while cutting in on the cup, but after measuring me wrongly she assumed her job was done and didn't seem to want to help find the right size.
I left, giving it up as a bad job, and decided to try my luck in another - much larger - branch of Mothercare the following day. This time she told me I was a 38DD. I explained that there was no way I was a 38 around the back and, as the 34 I was currently wearing was okay on the back but too small on the cup, perhaps I could try a 34GG or H?
She laughed at me, snorted: "But I haven't measured you anywhere NEAR a GG" and brought me a 38DD. Deja vu. And I then had to demonstrate just how ill-fitting it was before, again, leaving empty-handed and on the verge of tears.
At Bravissimo, they measure you by eye and can tell what is boob and what is back, if that makes sense. Even if their bras are too big, you can get some expert advice on which shapes would be good for you, and know what size you really are. Far better than a rude sales assistant trying to tell you that whatever the tape measure says is gospel, even when you know it's wrong.
Sorry, I ranted on a bit there, but I've been so frustrated by Mothercare and their lack of help this week that I could have cried. And I'm still in my too-small underwired 34Gs. Argh!