Bless you OP, what a stressful thing to have happened 
. I don't know if this helps at all, but even if they had a basis for a legal claim/the company had done something wrong in terminating their contract, there is no way you would have been held personally liable, regardless of whether you were named on the claim, it was the company's action and so it is the company who would have had to pay up (but as you say their claim is groundless anyway). Also, even if there had been some mistake or you'd done something wrong (which you haven't), I really doubt you would have lost your job over it particularly as the decision was made with another person, unless you were grossly negligent or for some reason of your own you decided to vindictively/deliberately get the company into trouble it wouldn't/shouldn't have been a dismissal matter, mistakes happen and any decent/reasonable employer will work with people to make sure they don't happen again rather than randomly sacking good employees. So in the nicest possible way I think your reaction is a bit out of proportion to the likely consequences of this admittedly upsetting event - are you feeling better now? If the sleepless nights and distress goes on would you consider seeing your GP or seeking some counselling to help you manage your feelings?
Don't get me wrong, I do understand the stress of getting a letter threatening court action personally addressed to you, I've been named in several court cases against my employer (employment tribunals) and have had to appear in court as a witness, been cross examined by an opposing barrister etc. (so far never lost a case!) and it is difficult. But at the end of the day I think you have to take a deep breath, acknowledge that usually it's nothing personal, there are plenty of people out there that have a useful sideline/make a little extra money by magnifying any little incident or adverse event that happens to them and threatening the 'responsible' company with court action - usually using the most extreme and dramatic language possible, claiming they are owed terrifyingly huge sums of money which is the most effective way to get people to cough up even though it has no basis in reality. If you think about it, if they lose 20 contracts this year and write inflammatory threatening letters to all of the companies (costs them 0), 15 of the companies will probably ignore them entirely, 4 will get their solicitors to write backing refuting the claim (as your company is) but 1 may panic and agree to 'settle' the spurious claim for a fraction of the amount claimed, thinking they are getting a bargain - voila money for nothing for the dodgy suppliers - it's basically a scam!
So I guess you can gather from that, that since this has been going on forever, no you can't really counter-sue people for raising a groundless claim against you, either for the personal distress caused or the costs accrued in getting your solicitors to respond (where I get these spurious types of claim in I usually save some money by simply replying 'fine, see you in court' and only getting the company lawyers in when they actually bother putting their claim in). Basically the way the law sees it, you can't sue people for suing you because this would potentially impair justice by deterring genuine claimants and could potentially end up in an infinite loop - can they then counter-counter sue your counter suing, and can you then counter-counter-counter sue them in response and so on forever? I guess if the language in their letter really was genuinely intemperate (making adverse comment on you personally, using offensive language, personal threats etc) there's a world in which this could constitute harassment which is a criminal offense, but if it was just strongly stating their case against the company I think you'll struggle to get anything out of them. I guess you could play them at their own game by threatening to sue them for megabucks for distress/offense to feelings etc even though you are highly unlikely to have a claim, but I'd be wary of dragging your company into this to be honest - they probably won't want their dirty laundry aired in public and if you were thinking of making some kind of public campaign against the supplier there's a risk of that going very badly wrong.
I think take it as a victory that they've achieved nothing by their posturing other than to ensure they are never ever getting any business from your company or you ever again, and try to move on with your life having maintained both the moral and financial highground?