Since all your contracts and documents you previously signed never mentioned that you would have to pay the training fees, I would highlight this to your line manager, and clarify that you are not signing any new contracts that now add this information (as per the suggestion of
@marple). You must do this (email or letter is better), so you confirm you do not agree with the new terms and conditions of any contract except the original, and that your employer must stick to the original one if they want you to work there.
It's normal for new employers to insist you have a new DBS completed, as a criminal offence could have been completed after you had your last DBS, though you can get automatic DBS updates online nowadays to avoid this sort of problem, via a paid subscription. However, it's not fair for you to be liable for the costs of training if and when you leave, as you took the job based on the information provided at the point of recruitment and the contract you signed. They have to make that clear in the original contract, not after!
If your employer is not understanding after you explain that you are not willing to sign the new contract because your original contract does not make such stipulations, I would suggest following the advice on this website:
https://www.citizensadvice.org.uk/work/rights-at-work/pay/problems-getting-paid/To try and resolve this issue, you can speak to a trade union for advice and mediation, raise a grievance with your employer, or resign, as they cannot recover training costs from you if it's not already in your contract. If you want to resign, make sure you give the appropriate notice based on your original contract (usually 4 weeks), including if this contract requires you to continue working/ training new staff within the notice period. However, it is unlawful for your employer to withhold your salary because of your resignation, unless you breach the terms and conditions of your original contract, or for example, they deduct it due to the PPE you took from the company. I know you said it was a mistake, but the latter is a reason employers can withhold salary. Regardless, if they do withhold your salary without proof of either one, you can sue them:
https://www.peninsulagrouplimited.com/guides/employer-withholding-pay/However, I am sure your employer would rather not go down this road. Before and during your resignation, be sure you keep an eye on exactly how much you are owed, and try your best to be professional and avoid breaking the employment terms of your original contract. Give back the PPE you took if possible and make sure everything you and your employers discuss are documented in writing (email or letter), which are legally admissible in case the worst happens.
Hope this helps.