Based on your experience do you think I should attend?
One possibility to consider would be to prepare a written statement and read it out to them at the interview.
Do not answer any questions - other than to say no comment because there is a danger you will contradict what you put in the statement.
The written statement simply being a version of the generic advice that is given on this (and other) thread as to what to say when receiving a letter from a TOC.
I do wonder why this TOC is going down this route of an interview given the time and cost they incur? Perhaps it is a way to try and test the sincerity of the passenger when they say they have "learned their lesson" / "wont do it again"? Perhaps it is to gain more insight into why passengers do not purchase the correct / valid ticket etc?
Mods- do you wish to split the latter comment into a new thread?
EDIT ADDTIONAL COMMENTS
I have given this more thought and wonder if
1) The purpose of the interview is to give SWR an opportunity to reinforce the message re wrongdoing whether this is openly or just through the stress the OP will feel attending an interview.
2) If the OP employs a solicitor the cost of doing so is a further "financial sanction" on the OP.
3) Furthermore employing a solicitor demonstrates to the TOC that the OP appreciates the seriousness of their situation.