In my experience, no one has ever turned down a tip (big or small) from me in any country. Except one time in New York (go figure). I know it may not be "expected" in some places, but coming from a service industry background, I can't "not" tip. It's physically impossible for me not to tip anything.
Yes, there are big differences in the UK depending on type of place - varying from 12.5% to 0%.
Many places, particularly in London, will add a "service charge" to the bill. This is the tip and nothing further is required or expected. Almost invariably in London, it will be 12.5%. Bear in mind it is always discretionary and you may choose to have it removed from your bill.
If a restaurant is going to levy a service charge, it will always be mentioned on the menu (usually at the end of the page)
Elsewhere in the country, the service charge is not so common and traditional tipping is more the norm. Again, a tip is always discretionary and many people may not offer one, or may offer one at less than the usual rate. Outside of London, the usual rate for tip or service charge is around 10%.
The above relates to "restaurants".
In cafes, tips are not really expected these days and, even if offered would only be a couple of coins. I cannot recall when I last tipped in a cafe - perhaps I never have.
Pubs can be tricky for foreigners, with regard to knowing when to tip and when a tip is not needed. To my mind, much depends on the eating arrangements. If a pub has a separate eating area, divided off from the drinking area, where serving staff take your order and bring your food then it is effectively a restaurant and I'd think about tipping as if I was in a restaurant. Similarly, the full-blown gastro pubs are, effectively, restaurants - nobody is going there for a quiet pint.
At the other end of the scale, at many pubs, the food is laid out on the bar and you order and are served from here - tipping is neither required nor expected in such circumstances. The tricky bit comes somewhere in between - the places where you're going to eat in the bar area, order food at the bar, but someone brings it to you. This is the most likely scenario you are going to encounter in a British pub. There might be circumstances where you feel a tip might be appropriate. I'd take the view that these circumstances are very rare - and , again, I cannot recall when I last tipped.
The norm (for Ireland anyway) is to tip 10% to 12% in a restaurant. Some restaurants add a service charge automatically, altough this is usually only for larger tables (say 6+), and this will be mentioned clearly on the menu where this is the case. Very very few add an automatic tip for smaller tables.
For cafes and bars it depends - you can tip few euros (up to 10%) if you are served i.e. if someone takes your order at your table and delivers your food and drinks. If you order at the counter in a bar there is no need to tip. Bartenders are paid reasonably and generally won't expect tips. Waitstaff generally will.