If you are on foot then it is even easier. From T2, follow signs for London underground and get on the Piccadilly line towards London. Get off at Hatton Cross, leave the station and you will be pretty much underneath the arrivals stream for 27L. Takes no more than 10mins and I think it is still within the free travel area - may be best to speak to a member of staff to see if you can leave the station as it may be only an interchange area. If not, will cost you a few £s for a return.