If you cross the TMA then you will need Mode S. If you fly airways in the UK the vast majority of the routes you will fly will take you accross the TMA.
Even if you file a route outside the TMA which is pretty difficult going anywhere south you still have a very high probability of ending up in the TMA. I would guess that I have probably had 1% of my airways routes as files around/across the TMA, the rest have always been vecotored. So pretty much if you are flying real IFR you will need mode S this month.