The issue here is that you are getting company sick pay, which is not a legal entitlement, but something which you get under contract. It is basically a voluntary payment given by the employer as a benefit to its employees. Regardless of what the contract says, if the employer believes that you are not genuinely ill, they can stop company sick pay. In their eyes you may either not have been sick or have recovered and stayed off sick for too long, simply because they were aware of you doing some voluntary work in a shop. Obviously, it does not look great that you are supposed to be off sick but are well enough to go and do other work. However, it is not as simple as that – you are off sick if you are unable to perform your duties with that employer. That does not mean you have to stay at home and be bed-bound for the whole time. People who are off sick can still do other things – take a walk, go to the cinema, go on holiday, etc – the main thing is that they are unfit to undertake the work which they are contracted to do. If that is the case, you are still able to do other things without having to risk your sick pay. Still, if the employer has doubts over the sickness absence being genuine, they can potentially stop your sick pay.
However, the way they have gone about it is not fair – they should have warned you and potentially given you notice that they are going to do that. They could have also tried to recover any overpayments of sick pay from future wages, rather than just stopping your pay completely and leaving you with nothing. That in itself can amount to a breach of trust and confidence and even unlawful deduction of wages. You should raise a grievance over this with them, after that you have to consider a potential claim in the employment tribunal (for free) if you wanted to pursue the money they stopped.
Please take a quick second to leave a positive rating for the service so far by selecting 3, 4 or 5 stars above. I can continue answering follow up questions and in particular can also discuss the steps you need to follow if you were to take this further. There is no extra cost for this - leaving your rating now will not close the question and means we can still continue this discussion. Thank you