First, absolutely agree, do not go to mothercare. Many sources will recommend getting fitted a couple of weeks before due date. I, from personal experinece, disagree - your rib cage is likely to get smaller and there's no telling how big your boobs will get. At least don't go all nuts buying up the whole shop, as the bras might not fit you for too long.
So I would recommend - measure yourself, get a couple of soft stretchy bras to start with that would accommodate several sizes. (like Royce Blossom). After your size has settled down, if you still want a personal fitting, go to Bravissimo. Or just use the fitting guide here: thebetterbracampaign.blogspot.com/ , order online and try at home - just order from a place offering free delivery and returns.
But now the most important question - sure you even want to torture yourself with nursing bras? Even the best non-underwired nursing bras (and I've tried many) will not give you the same support and you are likely to end up with droopy monoboobs.
Contrary to popular opinion, it is actually totally fine to wear underwires during breastfeeding - unfortunately there are very few underwired nursing bras available, especially in bigger cups. So my solution was to get my normal Freya, Fantasie, Curvy Kate etc bras cheap from ebay and convert them to nursing bras myself. You can find instructions and clips online.