The kick off is 16.45. It should be done by 18.45. Even with extra time and penalties it should be done by 19.30.
That's easily enough time to get back to Paddington for 21.00 easily if the game is done by 18.45 and still time if there's extra time etc.
To get easy access to Wembley I'd recommend staying somewhere near Hemel Hempstead. Get the train to Wembley Central. It's far easier to get away after the match if you aren't travelling back into London.
I'd suggest ...
KO 16:45 will be 16:50 earliest
1st half ends, after all the pi55ing about, 17:45
2nd half KO 18:05
2nd half ends, after all the pi55ing about, 19:00
10ish minutes of faffing about
1st period of ET KO 19:10
1st period of ET ends after all the pi55ing about 19:30
5 minutes of faffing about
2nd period of ET KO 19:35
2nd period of ET ends after all the pi55ing about 19:55
10 minutes of faffing about
Penalties start 20:10
Finish 20:20 and that's before the presentation and celebrations if we win
I may have gone a bit OTT but I'll bet my mortgage it'll finish way nearer 20:30 than 19:30 if it goes to penalties - absolutely no chance of a 21:00 train