Absolutely no soap or any sort of soaking in a bathtub with anything other than water in it. If showering, just let jet from shower run over genital area for a couple of minutes.
It is so important to keep very hydrated, particularly if experiencing symptons of a UTI. If caught earlier enough can, sometimes, even literally flush it through.
Surely, you do not need a GP appointment to get a 'sample' tested. Most GP surgeries have arrangements for these to be dropped off and sent off to local labs for testing. You are fortunate in that you seem to be able to deal with your UTI's usually by drinking load of water and not requiring anti-biotics.
Sample needs to be tested as different infections respond to different ab's. My worst time with these was when for four to five weeks I had constant UTI's - six different prescriptions for anti-biotics, But as fast as as one infection started to clear the next one commenced. I was quite unwell all that time, two visits to emergency departments. Finally sent to urology where camera showed a growth in my urinary tract = with a couple of weeks I was taken into hospital to have that removed and checked. Was benign. Since then I have to have annual checks to see if has started to re-grow - but -thankfully, only one UTI (at a time I was very stressed).