I will try to clarify as much as I can with PCS before the PC is dispatched. In the event that I have to pay VAT again and for example PCS refunds the UK VAT, I would still end up losing money, simply because the import VAT would be higher than UK VAT, like take the PC costs 2500 euros without any VAT, I pay 20% UK VAT, which would be 500eu, so the total price/value of the item that comes up to the customs is 3000eu, then I pay 21% of 3000, which is 630, PCS refund me 500eu, I end up losing 130eu.I would hope, and TBH expect, that if you have to pay VAT on import that PCS would refund you the UK VAT. The process to get that refund and then get it to you may be a long one however because this is all so new.
I do understand your anger here but companies have to follow the tax rules that are in place at the time. When you placed your order nobody knew whether there would be a trade deal nor what the terms were likely to be, so there would have been no way PCS could have advised you of anything. And as I've mentioned, at the time of your order they were under a legal obligation to charge you VAT.
I would suggest you wait and see what happens when it gets to you. If you have to pay VAT on import I would then contact PCS asking for a refund of the UK VAT you've paid (with proof of VAT paid on import of course).