You can get some amazingly cheap clothes, but they can be very poor quality, too. There's also ethical concerns about how they are sold so cheap: the workers barely make enough to stay on the breadline in a lot of these factories.
I'd order using actual measurements rather than clothes sizes, as their sizes do tend to be smaller, and they aren't often labelled correctly. It's also tough to return things!
I've bought clothes for DD from China/Hong Kong - on ebay- and whilst they have been lovely, and incredibly good value for the price (I bought party "Princess" type dress and a gorgeous embroidered dress in a "Chinese Lady" style) they were VERY small fitting. I think she was 8 and then 9 when I bought them (for Christmas each time) and I bought something like a 13 and a 14 years and they each just fitted her, and she was in normal 8-9 and 9-10 clothes from ordinary shops. I would go for BIG. The only exception is Holy Clothing (american company but their stuff is made in and despatched from India to order, so actually gets to the UK faster than it would to the USA) and their stuff tends to be generously cut and sized - a 4XL is about a 28-30 from Evans. Just watch out for their shipping people invoicing you for taxes after delivery - it can work out cheaper to order one or two things at a time rather than a larger parcel as they hit you for import duty and it's something like 20% iirc. Oh and they once (out of about 6 times) charged my card twice for the same order so keep an eye on them
Chinese clothing is usually very good quality. Ebay opened up a new avenue for bespoke retail. Before ebay, tailors would come over and take over suites in hotels to conduct their business, measurements all faxed back to HK and bespoke suits delivered airmail 2 weeks later with tailor still in situ to make minor adjustments.
Ebay opened the door, but I suppose it can only be hoped you can measure yourself properly.
Several friends have had wedding dresses made at well under £100 that you would pay the thick end of £1,500 for if made in this country.