I’ve had a similar situation occur.
My experience is that making inquiries with Camden Council results in either no reply (as in your case) or as in my recent situation where I initially had no reply – prompting me to complain – and only then did I receive a partial and unclear reply, followed by an unexpected action – the removal of my property.
Regarding your case. I believe that as leaseholders we do have a responsibility to keep walkways clear. The Council may try to argue that you’ve not done this by placing your pots and table where they were. However, as you’ve said – they’ve been there for a decade – and undoubtedly Council employees have seen this – as well as your neighbours – both of which had ample time to speak up to indicate their belief that this was an obstruction and request that you remove them.
I would think, as it appears you do, that an invitation for discussion sets out a process for correspondence and resolution. Which you attempted to follow. By not responding – and simply removing your property, it could well be argued that the Council has either committed a criminal act (theft if they do not return your goods – or criminal damage if they return them damaged). If reported to the police, they may agree to lay these charges. If they don’t, then it may be that you have a good case for small claims court.