Just checked with an AAC pilot he says its still the M60D on the ramp sometimes a .50
UK Chinooks are only certified to fly with M136 miniguns (doors only) and/or M60D. There is no mount in the RAF inventory for the M2 or M3M .50 to fit the Chinnok door or ramp.
GPMG/L7AX aren't certified for use on Chinooks. One of our tech advisors is a Chinook Loadmaster, we've had all the gen off him with pics and a guided tour. There is a huge back story to the M60's, i dont remember it all but it revolved around a deal for the Chinooks complete with miniguns. They didnt have the miniguns available at the time so we ended up with M60s as a stop gap. They proved to be so good they didnt get replaced with the latest version of the GPMG. Now they are a legacy item its probable they wont be replaced until they die.
When Dave gets online i'll get him to post the full story if you like.