It needs treating ASAP. UTIs are bitches of things and won't go away on their own. Needs antibiotics, and see if you can get your hands on something like Ural to alleviate some of the symptoms until the antibiotics kick in.
Yes call 111 for an out of hours appointment. You do need to drink as much water as you can - firstly because you will need to provide a urine sample and secondly because if there is an infection it will help flush it out. It might just be pregnancy hormones making you want to wee all the time but as untreated UTIs can cause problems in pregnancy, you don't want to take the risk. If you weren't pregnant you could maybe take your friend's advice to drink water and cranberry juice and wait it out, but given that you are pregnant it's irresponsible advice tbh.