In Britain it is quite normal to tip taxi drivers and it's customary to round the total fare up to the nearest pound. If you choose to tip your taxi driver because they help you with your luggage or have delivered excellent service, a 10-15 per cent tip is a nice gesture.
Although you don't need to tip a delivery person when they drop off your food, an extra pound or two will always be appreciated. If you're in a restaurant a service charge is sometimes added to your total bill, which you are expected to pay. If nothing is added and you are happy with the service you've received, a tip of around 10-15 per cent is standard. Many restaurants automatically add a service charge to the bill, so do check before leaving a tip.
If a porter takes luggage to your room for you, it is customary to tip them approximately £2. If you decide to order room service you can leave a tip at your own discretion.
If you get a haircut while in the UK it's polite to tip the hairdresser approximately 10 per cent. If a junior member of staff washes your hair it's nice to give them a couple of pounds too. If you are in any doubt approximately 10 per cent for any good service you receive, will always be appreciated in Britain.