I know you can take the pet in the cabin on LH flights in mainland Europe as I have done it with my small dog.
The UK has fallen into line with the EU Pet travel rules so they are now allowed to travel between UK-EU without quarantine (and provided they have all their shots and passport).
HOWEVER going by this link
http://www.heathrowairport.com/plan-...ling-with-pets
I believe LHR still require them to go in the hold where as the LGW website states that some airlines allow pets in the cabin.