Question: This question is based on travelling between the UK and Cancun, Mexico.

On the way out (UK -> Mexico) I purchased 200 cigarettes in the duty free shop at LGW. Some (at the current rate c. 50% by the time I fly back in 6 days) will have been smoked, leaving a remaining quantity. Will these count towards/against any duty-free allowance I have when travelling from Mexico back to the UK, or will I only be entitled to purchase/import my return allowance net of the amount I have remaining?

Answer: 
No, the limit is a total amount you can bring in, regardless of where or when on your trip you bought them - so if the limit is 200, and you've got 100 left from your outbound trip, you can buy another 100 on the way back to take you back up to 200.
I've previously been interested in a related question, which is if I buy an item tax free at the airport, such as a laptop or phone, am I supposed to declare it when I return home? I found a link that seems to answer both questions:
<http://webarchive.nationalarchives.gov.uk/20080814090357/http://customs.hmrc.gov.uk/channelsportalwebapp/downloadFile?contentID=HMCE_CL_000282>

2.1 Am I entitled to any duty-free allowances?  

Yes. In addition to any of the reliefs in Sections 4 to 8 for which you may qualify,
when you are travelling in from outside the EC you can have the relevant allowances
for alcoholic drinks, tobacco products, perfume, toilet water and other goods,
(popularly known as 'duty-frees') shown in our Notice 1 A Customs guide for
travellers entering the UK. *These allowances apply to goods obtained outside the EC
or bought duty or tax free when leaving the EC, and which have travelled in with you.*