I will be travelling to London for the first time at the end of July. I have read the posts about tipping at restaurants, concierge, etc., but couldn't find anything about tipping the porters at the airport (Heathrow) if there are any. Will there be a porter to help with my bags through customs and out to a taxi service? If so, is there a customary tip for this service? Should I convert money before flying over from the US to do this with?

I've never seen a porter at baggage carousels in any airport. There are trolleys you can use for your bags to take them from the carousel to the terminal exit and taxi stand.

Forgot to add that you may have to pay for the trolleys and you will need local currency.

You don't have to pay for the trolleys at Heathrow when you're arriving and collecting luggage before you pass through Customs (you've already been through Immigration at this point). As for a taxi service, follow the signs once you're out of Customs. Only take the black cabs,(in the official taxi rank) the mini-cabs (unmarked) can be a bit of a risk if you don't know how much you should be paying for a journey. A taxi from Heathrow to central London will be around GBP 55.00, so Heathrow Express to Paddington station can be a better option, depending on your final destination in London.

Never seen porters at any UK airports, maybe you can pre-arrange one if it's important to you.

Taxis hired at the airport are extremely expensive, the Heathrow Express is rarely the best way into London as you generally have to use another form of transport to get to your final destination.

In most cases the Tube (cheap) or a car service (cheaper than HEX for two or more people) are the best options. In some cases taking a coach to Victoria might be the best option.

A taxi will be expensive, better to have a pre-booked car service like Just Airports: http://www.justairports.com/ or Simply Airports: http://www.simplyairports.co.uk/ and a half a dozen others that are often mentioned on this forum. Even cheaper to take the Tube.

I have used a porter at Heathrow to assist with my luggage. They are near the carousels. I tipped them £10 for assisting me with my suitcases to the taxi

You really need to book a porter at LHR if you need one: I've hardly ever seen them hanging around - or anyone seeing the point of them

www.skycaps.com if you do need one.

"tipped them £10 for assisting me with my suitcases to the taxi"

I bet they thought Christmas had come early, or was it at Christmas??

Porters??? I've been through Heathrow dozens f times and never seen one. How much luggage do you have? (Any reasonable amount will fit on a trolley.)