As far as the UK is concerned we come under NATO's Active Layered Theatre Ballistic Missile Defence (ALTBMD) programme that is supposed to provide complete coverage for NATO countries against tactical ballistic missiles with ranges up to 3,000 km. We are also linked into the US GMD system for ICBM defence.
There are two Ballistic Missile warning sites located in the UK and they are linked into this ALTBMD system as well as the US GMD system (The GMD system can intercept ICBM at ranges out to 10,000 km
ALTBM Interceptors are based around the Aegis system deploying SM3 BIIA missiles based in Rota in spain (4 x US Aegis destroyers) - other interceptors include sites in Turkey and Romania (these are land-based Aegis sytems). Other land-based Aegis sytems are due to be installed in Poland. Not sure about how the US GMD system is setup.
I think BMD (Ballistic Missile Defence) is the one of the main reasons Putin kicked off over Ukraine as in theory it blows the principle of MAD (Mutually assured destruction) out of the water. By this I mean that an aggressor who has a BMD system could in theory launch a nuclear first strike on an opponent. Once the opponent sees the launch of nukes it would immediately launch its own missiles back (MAD). However the aggressor here employs his BMD to knock out all the incoming nukes in low earth orbit - ergo, the theory of MAD no longer exists.