If you ever go to another part of the world, like India for example - would you take a bin bag and tidy up all the 'rubbish' offerings at their sacred sites too.
Or maybe thats different? not 'British'?
Traditionally, in India, it is food that is offered to the Gods. This is of course 'biodegradable'. If all offerings were of 'biodegradable' natural material there would not be a problem with litter louts.