Haven't heard of DX, Mariam. But there again, Myrna came here as my spouse in March 2009 after applying for her visa through the British Embassy in Manila. And ... ... since she received her visa through
a delivery service called 'To Go' ... there's no reason why I would've.

Would imagine, though, DX seems likely to be the carrier the Home Office uses. However, research on Google yielded only the following telephone number:-

01604 782 882 ... which may or [equally] may not relate to this Company (*could even possibly be a subsidiary!) *that, I'm afraid, ISN'T altogether clear, alas ! ... but it's definitely worth a try.