Alkin is rarely used for a hold - unless the aircraft are close in. This is because it blocks the 28 approach, and any traffic inbound form the north via LAM.
Some people like to hold aircraft at the LCY NDB. usually at 2000ft. Again, depends on the situation as it blocks departures. There are many ways to do it depending on the may variables in the equation! (ie if the wx was close to accpetable limits, then holding close is better so you can dive them in before it detiorates again - the outer holds are miles away. Also, maybe the TC South East Low secotr was becoming overloaded, and the friendly Thames controller brough them into the Thames airspace to help the TMA out.
ALKIN only has 3000ft and 4000ft anyway - so aircraft are held further back at SPEAR or DET or even as far out as BONDY.