Just arrived in Cambridge, UK for the first time today and when I went out for lunch (at Revolution), they put my card in a handheld machine but never asked/provided a way for me to tip. I've read U.K. isn't as tip mandatory as the US, but what is the proper way to tip with credit if they just take your card and run it? (I kinda thought they would hand me the small machine and it would have a place to enter a tip)
You can ask them to process an amount more than the bill itself or, more normally, leave a tip in cash. In cash is more likely to find its way to the people serving you, rather than management. Often also no tip is expected because a service fee has been added anyway. Cash may also be appreciated as a means to evade some tax.
Sometimes the bill does have a place for a tip and is left for a little while with the customer to review and possibly find the right amount of cash. It is only on a later visit that either cash or card is collected.