If they are in China, I would NOT expect a cheque drawn on a UK bank to be that much use, because the clearing system doesn't work - they cannot bank it, it would have to be returned to the UK and the funds cleared into a UK based account of the beneficiary, which is clearly impossible, unless the Chinese company has a UK arm. The Post Office web site also confirms a postal order is not acceptable in china - there is a list of countries on their web site but china is not on it.
If you have a current account, then your bank can transfer money to the chinese company via Swift, or Iban - there is a charge for this, which is quite high sometimes - £25-35 seems typical including currency conversion. If you really want to buy from China, and they don't have credit/debit card facilities (which are only usually used by the small businesses) then you need to use proper business services - which require you to maybe even have a business account. I paid over a grand today to one Chinese company via debit card, but another for just £200, meant a proper transfer and a £25 charge, using Swift.
Big chinese companies do NOT take cards - that isn't how it's done.