The last one I've been to had a piece of paper taped to the door, with "Please leave this toilets (sic) as you expect to find it!" written on it 

Im confused, how does strengthening your urinary tract make your foof tighter?  (Or give ability to tighten nbetter) if theyre different holes?

From the NHS website.

The pelvic floor muscles are located between your legs, and run from your pubic bone at the front to the base of your spine at the back. They are shaped like a sling and hold your bladder and urethra (the tube urine comes out of) in place.
The pelvic floor muscles support the bladder and bowel and give you control when you urinate. They relax at the same time as the bladder contracts (tightens) in order to let the urine out.

I have been doing my method since I can remember. However the NHS advise against my method as it may be harmful to the bladder (which I didn't know until now)

This is what the nhs recommend you do:

How to do pelvic floor exercises

You can feel your pelvic floor muscles if you try to stop the flow of urine when you go to the toilet. However, it is not recommended that you regularly stop your flow of urine mid stream because it can be harmful to the bladder.
To strengthen your pelvic floor muscles, sit comfortably and squeeze the muscles 10-15 times in a row. Do not hold your breath or tighten your stomach, buttock, or thigh muscles at the same time.
When you get used to doing pelvic floor exercises, you can try holding each squeeze for a few seconds. Every week, you can add more squeezes, but be careful not to overdo it and always have a rest in between sets of squeezes.
After a few months, you should start to notice the results. Your incontinence should improve, as well as the sensitivity you experience during sex. You should carry on doing the exercises, even when you notice them starting to work.

I actually cannot do this method without getting horny and needing to flick the bean although now I think about it might be worth doing before an appointment.