Welcome to the forum!
The principle here is that purchasing tickets isn't illegal but using them illegally is. Greater Anglia have conducted a data trawl that indicates a journey pattern that doesn't look right and have contacted you about it. The issue is can Greater Anglia link these journeys to you. There are two possible outcomes here:
1. You co-operate with Greater Anglia and 'fess up' to what you've done. They will almost certainly offer you an out of court settlement. You won't be prosecuted and won't have a criminal record. A settlement is likely to cost the sum of all the fares avoided at the full Anytime rate with no credit given for the invalid tickets you did purchase. In addition you will have to pay an administration fee, typically around £150.
2. You say nothing. You aren't required to incriminate yourself. It's is unlikely (but not impossible) that Greater Anglia will have sufficient evidence to link the suspicious data pattern to you, to a standard of proof required to secure a conviction in the Magistrates Court. It is possible that Greater Anglia has CCTV or some other evidence that links you to the suspicious journeys. If Greater Anglia want to prosecute you then they must lay papers before the court within six months of the offence.
If you decide not to engage then Greater Anglia will send further letters threatening police action and prosecution for fraud. Also, if do decide not to engage then do not contact them and ask for more information as any sort of engagement could be seen as incriminating.
If you co-operate then once the settlement is paid that is the end of the matter. If you decide to say nothing then it probably won't go anywhere but it might do, either now or in the future.
It's really down to your appetite for risk.
