Although there are now through Thameslink trains to East Croydon from our local East Coast Main Line station at Peterborough, this was in the early days when cancellations were rife and the service very unreliable; further, it is still quicker to get a LNER train to London Kings Cross then cross the road to St Pancras International and take the next train to East Croydon - and a far more comfortable ride, too. When we arrived at St Pancras there was only one train per hour going through to East Croydon, and it had not come from Peterborough! I think they are running properly now but it is probably still quicker to take a fast train and change.
We have Oyster cards so we never have to book beyond London now, using our Oysters for onward travel to Croydon or elsewhere. Transport for London now accepts contacts credit or debit card payments on its trains, trams and buses and if you use the same card all day the daily cap will be applied just as if you are using an Oyster, so on the face of it a visitor is just as well off using an ordinary credit card as going to the trouble of acquiring an Oyster, but for us as Senior Railcard holders, the big advantage is that we can link our railcards to our Oyster cards and receive the railcard discount (one third) off our London travel.
We travelled from our hotel to the party and back using the excellent Croydon tram service, and again for church and lunch with our friends on the Sunday, then travelled back into central London to get our train to Peterborough and home to Stamford.