When I visited a couple of years ago all I had to do was to show some photo id (your passport would be fine) and some other id, my drivers licence was fine. Then she filled out a form on the computer and took me through some rules then a machine took a photo and a short while later the card was ready for me to collect at the desk. In the time whilst I was waiting I browsed some information on the computer and planned what I was going to look at. I did find some good info but I wish I'd planned more.
Be aware that getting there may take some time. The journey from central London takes around 40 minutes then there is a 10-15 minute walk (depending on how spritely you are) to the actual venue. There is a large canteen and bookshop on site so there is no need to stop to eat before hand. Once there you need to put any bags and coats in a locker and take limited materials, paper/pencils (no pens), laptop etc into the research rooms.
It is very wise if your time is limited to be REALLY sure before you arrive what you want to look at, so that you maximise the time you have there. It is worth spending time on their site before you arrive finding out what's available and deciding what you want to look up. It can be really frustrating to not know where to start and end up browsing their website thinking, "I'm wasting my time here I could have looked at their website from home and been organised".
Unfortunately I fell foul of this and found that I wasn't that sure what I was wanting to see. I ended up looking at their computers and was disappointed to see that most of what they have on the internal website is simply links to subscription access to Ancestry or other such sites. I felt that if I'd known what records they had I could have gone with a definite plan. Instead I spent the hour or so after I got my reader card simply finding out what I could see then realising that a whole lot of what they had wasn't much more than I could get online anyway. I ended up after 3 hours having something to eat buying books in the shop (some really varied books there I've not seen before) and heading back to London feeling I'd wasted my day because I wasn't prepared.
So if you plan to visit I'd recommend you plan to make a morning or an afternoon of it as its a hour there and hour back travelling if you are staying in central London. Thus it seems a shame to spend only an hour or two there. Be well prepared in advance, know what you want to look up, or who you want to look up, plus be aware that time flies quickly whilst you are there its easy to get engrossed in what you find. Also nearby is the world famous
Kew Gardens so you could make a day out of it, or offload anyone accompanying you to the gardens whilst you browse dusty documents!!