Tickets for places like Westminster, Churchill war rooms ect

Late September is still busy season in London, in fact we were just there the last two weeks of November and it was crowded. It's advised to prebook for Westminster Abbey, especially if you want to go in the morning. We had tickets for 1:00 and there were no lines. I highly recommend you spend the extra £5 for the Queen's Galleries.

Tower of London was jam packed at 10:30 in the morning on a Sunday but had cleared out by 2:00. Depending on your interest you may want to pre-book for the morning though because there's a lot to see. We spent 6 hours there.

I've never been to the Churchhill War Rooms but the one time I walked by there were two long lines; one with timed entry tickets and one without.

For the Churchill War Rooms you should book at least a few days ahead of time. There is no need to book for the Tower. the best plan is to arrive about 10 minutes AFTER the scheduled opening time. By that time the queue of those who arrive early will have gone and you should be able to walk straight up to the ticket office. Then go straight to the Crown Jewels, see them before it gets busy and come back for the next free Beefeater tour.

If the answer to the question "will I regret not seeing xyz because I didn't have tickets" is no, then take your chances. My view is if you really want to see something and you have limited time, then book the tickets ahead.

When we visited the tower, abbey, and war rooms a couple years ago it was in November and there was a very short line at the abbey and no line at the others. We tend to take our chances getting into a place rather than be on a firm schedule.