Is it possible to ignore the letters as they're not registered post, saying we never received them ?
No - lying to try and escape liability for an alleged debt is a bad idea. At best, it won't work. At worst, he would risk turning what is currently a civil matter into a criminal one.
There is no requirement for them to send letters by registered post - a letter sent by regular post is presumed delivered 2 working days after sending, for the purposes of relevant legislation.
The good news with Euro Car Parks is that, although they will probably eventually take the matter to court, they will likely use DCB Legal to do so. In our experience, for a single ticket, where a DCB Legal charge is defended, they pretty much always discontinue before the matter reaches court. So, if your son follows our advice, he's a very strong chance of winning.
The important thing however, is that he'll need to open all his post promptly, or make arrangements for it to be opened for him. Letters sent by debt collectors can be ignored - if/when he receives a "Letter of Claim", you should come back here for advice.
