I am UK based and worked for a company for only 2 months, the job was not what I was expecting but I still carried on working there and didn't miss a day and completed all my tasks correctly. I was then, unexpectedly, offered another job that was much more suited to my career path. My contract stated that I had to give one weeks’ notice so I submitted my resignation letter on the Friday and the company agreed to accept this. However, on the Monday morning I tested positive for covid and as I am extremely clinically vulnerable, I was issued with an isolation note by the NHS. I sent this through to my employer on the Monday morning at 7am (2 hours before I was due to start work for the day). I heard nothing from him until the following day when he sent me a 'Termination of contract email' stating that I was terminated with immediate effect for not attending a client meeting on the Monday morning! he also stated that this was effective as of the Friday that I handed in my notice, and the same day that he agreed to my resignation. Fair enough I thought, he is just being petty and doesn't want to pay me for my weeks’ notice (I can deal with that as I was off sick and would not be entitled to sick pay for the first 3 days anyway) The real kick in the teeth was when I received my final salary pay yesterday there was no holiday pay included, I had legally accrued 4 days’ worth whilst there. I mailed my former boss to question this and his reply was as follows "I’m afraid you were let go from your position which means you are not entitled to paid holiday” I know this is illegal! Basically, I need to ask advice as to what I should do next? Please note I was not terminated for gross misconduct, just the fact that I missed a meeting whilst off with covid, and I was terminated 4 days after my resignation was accepted. He is just being super petty and quite childish in my opinion and 4 days holiday pay is legally due to me. I replied to his email saying stating I was not due to holiday pay with just the following "No problem. I will take this further"
Citizen's Advice Bureau and/or ACAS is a good first port of call here. They'll be able to advise you and point you in the right direction. The ACAS Early Conciliation process is something that would be well worth a look IMO. It's faster than a full tribunal claim, doesn't cost you anything and still leaves you with the nuclear option if it doesn't work.
And if you fight them on this I'd say you're on pretty solid ground - as you say they aren't allowed to do what they're trying. Assuming your contract didn't include sick-pay they can not pay you for the three days, but after that the SSP rules come in to play (you would likely be eligible). And they can't dismiss you with "immediate effect" to get around this - you've been there more than a month so the statutory notice period is a week.
Even if you had been terminated for gross misconduct you'd still be entitled to your holiday pay - that's enshrined in law. You've already earned that money