If the librarian was on the phone, she was probably talking to someone who was renewing their books, making a reservation or maybe booking a computer. Probably someone older who doesn't know that you can or doesn't want to do all of these things over the internet from the comfort of your own sofa.
If you look at your council website, you'll find a link to your library catalogue, you can browse, reserve and then pick up your reservations on your next visit. It means that each branch can keep less stock, but still provide a decent selection.
If you live in London there's a fair chance that your borough is part of the London Libraries Consortium which gives you the choice of 14 boroughs-worth of books.
The library website will also give you access to things like digital newspaper archives, on-line reference books (encylopaedias etc) homework help, and even free on-line courses via "ALISON" and also unlimited access to the Driving theory test practice and the UK citizenship test practice.
Most libraries willl provide public computers and free internet access so that those without it at home can fill in government forms, do job searches, order their shopping, or browse facebook.
Most library staff would not interrrupt someone who was browsing the shelves, but if you go and ask for help they will spring into action and fire up the catalogue or give you personal recommendations.
There are now many fewer library staff. There is unlikely to be a floor-walker anymore, just a couple of staff behind the counter and a couple of self-service issuing machines.
Please go back to your library and talk to the staff, browse the local library website/catalogue and look at www.fantasticfiction.co.uk/ to get some more ideas of what you might enjoy before you go!