I think that most sites have squids of some type (be it for cvmfs or Frontier for ATLAS, CMS, or LHCb). Many sites (not just in the UK) have wondered if it is possible to combine these squids and from a load point of view this should be fine. However having not done this myself (as the Tier 1 keeps all it's services separate for the moment) I don't feel like I am in a good position to advise other sites what to do.
Therefore if anyone has tried sharing ATLAS and CMS Frontier squids, sharing Frontier and CVMFS squids or doing anything cunning with failover should their primary squid go wrong I would be interested to hear. I will be putting the useful information I compile on to a WLCG twiki page. Even if you tried things and it didn't work it may still be a useful starting point. ATLAS have asked me to test this so I am trying to learn from other peoples mistakes first!