First check your sub domain is not on the list for which they are allowed to fail to return a response, see http://www.noticeoflawsuit.com...
- if so, they are accusing you directly of hosting malware, probably best check that out first...
Otherwise, if you are not on that list then it looks to me like they are violating the order if they are failing to return response for your subdomain. You would need to collect evidence (failed dns resolutions etc.), and evidence of your costs (alternative service provision, trips to check security manually, don't forget to charge your time at normal daily rate... etc.) and then you could sue them - since they have UK presence. Class action is not an option (usually in the UK) but small claims is, although it costs, but it may be worth it depending on how much damage they have done you. Remember you are not challenging the US court order but rather MS failure to deliver continued dns service to subdomains not on the court order (as it implies they will). If you are UK based and since MS has UK presence, UK court is probably correct venue. If (and IANAL) this is feasible, then it would be best if lots of affected people did it at once - since like most courts MS has to file defence / turn up, or they lose, and if there are a _lot_ of cases all at once...
Before small claims court you are supposed to try and resolve the issue and there are some rules (letter before action, http://www.justice.gov.uk/cour...) - basically you can start writing complaint letters to MS in the UK (where you may sue) and cc MS in the US, and you can start costing them lawyer time right now, for very little cost and essentially no risk to yourself.
Or maybe talk to the EFF, they might get involved in the US because if MS have ceased service to legit users not on the list in the order and not hosting malware, then they may be in violation of their own court order. EFF might want to get involved at next stage and submit Amicus Curiae brief on behalf of innocent users.