I am struggling to see how you could have been delayed that much, but obviously you were! As you can see, having missed your flight by just ten minutes, allowing the recommended 2 hours would likely have let you make it even with the delays you experienced.There was no faffing, I assure you. Once I got on the train at Luton that was a slow one stopping for something on the track, it was betting down with rain that morning, branches ?. Once I arrived at St Pancras I then had to get to London Bridge (it's ok looking at timetables but when your stressed out and trying to get to your location and at the back of your mind your flight !) Once at London Bridge the train took an hour stopping at each station. All In all that took me almost 3 hours to get to Gatwick and still didn't make it. From 8-11 I arrived at 11.20 my gate shut at 11.10. If you are a regular train commuter then I guess you are use to the system and how the trains operate in the UK. I use them only now and again and on the whole they have been ok (we won't talk about how much it costs). I did the same journey over the summer no problem, I think that is where I ended up coming unstuck. You need to leave no end of time on a Sunday as it is way much going on. Including the nonchalant guard who told me to go to London Bridge. I had previously caught it from St Pancras to go to Gatwick over the summer. I'll try my best to see if I can at least get my one way ticket back, if not thanks for all the tips and advice from everyone.
For future reference you might also want to try and use Google Maps or another journey planner if your plans don't work out - they will suggest the quickest option in the circumstances. That would quite likely have involved going via Victoria rather than London Bridge.
I don't think you will get anywhere with trying to claim a refund for the new flight you bought, and in terms of a claim for delay compensation on your way to Gatwick, such a claim is highly likely to be denied on the basis that the timetable changed after you booked.
The difficulty you have there is that, whilst that might not be a defence/exclusion that necessarily holds up in Court, are you going to pursue it that far for the sake of £20 odd in compensation? Also bear in mind the fact that they may claim you delayed yourself unnecessarily and can thus only claim for a shorter delay.
I would never think to check my flight is running before getting to the airport. I know that the airline will inform me if it isn't - something which the rail industry seems to greatly struggle with.You'd check the flight is still operating before going to the airport, so why not the transport to the airport?