Just to afirm some of what has been said and fill in some gaps. If you have been with them less than a year, the only protection you have against dismissal is on the grounds of race, gender, sexual orientation, age or religious belief.
If it's simply unfair dismissal then with less than a year, there is zero protection.
If you've been there more than a year then they need to go through a formal process before dismissing you for poor performance. That usually takes around 6 months and involves them meeting with you formally (you have the right to have representation) and explaining what it is specifically they are unhappy about with regard to your performance. They need to then outline in detail what they expect you to be able to do and give you some measurable objectives/targets.
If they said that you have to be able to work with this new piece of software that they have introduced retrospectively and they don't give you training, then that sounds very much like constructive dismissal.
The call to ACAS or the Citizens Advice bods will be well placed and similarly any call to a partner in employment law will usually result in them listening to your situation and then saying whether there is a case or not. Aftter that they will charge you.
Good luck mate – it's a tough place to be in. Either way I would suggest you think hard about pursuing a claim as it is a long drawn out and expensive process unless you've got union representation (first time I've advocated that!) It may be better to just move on, it really depends on how long you've been there.
Bear in mind that the compensation for unfair dismissal not on the grounds cited above, is capped at £50,000 so no matter how bad they've been, that's the limit for what they can be sued for.