The British like queuing,but it's amazing that no formal queue is seen in British bars actually. The bar staff are skilled at knowing whose turn it is.You're permitted to try to attract attention,but there are rules to follow. Do not tap coins on the counter,snap your fingers or wave like a drowning swimmer.Do not scowl or sigh or roll your eyes.And among the do-nots lies prohibition of ringing the bell behind the counter---this is used by the pub owner to signal closing time.The key thing is to catch the bar tender's eyes.You can hold an empty glass or some money,but do not wave them about.Do put on an awaiting,expectant,and slightly anxious facial expression.If you look too contented and complacent,the bar staff may assume you are already served.
Do often say the word "please",and bear in mind about pet hates of the bar staff as far as possible.They dislike that guests are so hesitant that they have to wait for them;they dislike that many of guests are queuing to buy the wines while somebody are standing by the counter;they also dislike that someone say they would like to drink such wines as Guinness Stout in the end in that it requires a lot of time to prepare compared with some other types of wines.
As for the tip,you should not give the cash to the bar staff to show your acknowledgement.
The better way you do is to offer the bar staff a drink.A kind of egalitarian atmosphere has become the proudness and delightedness of the pub.A tip in cash will remind you of a serving bar staff,whereas offering them a drink is a friendly expression style.