Active? Who knows, the damn things break down all the time. I think we have a paper strength of a few hundred Challenger 2's, however most are mothballed, we've a 100+ operational and in all honest likely to cut that further.
A lot of companies are re-equipping with Scimitar light tanks which are an unprintable word and somewhat pointless as I doubt even those formations have an idea what they are supposed to do in a war. But there is a lot of politics over those blue blooded cvalry formations

At division and brigade level they are mixed. A battlegroup would be taken out of a brigade in theory, in reality it could be from a mixture of brigade.
The 4th mechanised Brigade consists of
204 Signal Squadron (Signals and Brigade HQ)
The Queen's Royal Lancers (Formation Reconnaissance)
The Royal Dragoon Guards (Armoured)
1st Battalion, Scots Guards (Armoured Infantry)
1st Battalion, The Duke of Lancaster's Regiment (King's Lancashire and Border) (Mechanised Infantry)
1st Battalion, The Mercian Regiment (Cheshire) (Light Role)
4th Regiment Royal Artillery (Self Propelled Artillery)
21 Engineer Regiment Royal Engineers
Here armoured is mostly challengers though increasingly scimitars. Reccie is a variety but nothing heavier than a scimitar. Amroured infantry is Warrior AFV's, mechanised is Saxon APC's, light is trucks and SPGs are those 155 beasts and rockets. The equivalent in Epic would be Leman Russ, Salamanders, infantry in Chimera, infantry in rhinos, dismounted infantry, basilisks and manticores.
Last time anything like this fought was iraq during the invasion and one incident in Al somewhere or other where they put a shed load of challengers and warriors through a town to shock, awe, lure the muppets there into fighting (it worked, a lot of Iraqi's obviously died for no loss amoungst the brits, they got a bit wiser after one more attack on the base, that or they simple ran out of young men who didn't know what to expect) and resupply the garrison.