Why should I pay in bitcoin as opposed to traditional payment methods? Recently I have seen a few online stores starting to accept bitcoin as a payment method, but I can't think of a reason why I would ever want to pay in bitcoins. 

Since I get paid in GBP then I would need to convert that into bitcoins somehow. Most likely by buying them from an exchange. Surely this just creates an unnecessary step since I could just pay the merchant directly using the same method that I purchased the bitcoins from the exchange with. 

Even if you are worried about anonymity you could just send cash through the mail directly instead of first sending it to an exchange. I mean, ultimately you need to convert the currency you are paid your salary into bitcoins one way or another. Why not just use that method to send the money directly?
This is an interesting question, and certainly one to which the answer will evolve before Bitcoin adoption could hit the mainstream.

On an abstract level, it is trivial to answer: In order for you to want to pay in Bitcoin, it must give you an advantage compared to other payment methods, or it must be the exclusive path to some good or service.

There are a number of concrete reasons, although maybe none of them will apply to you personally.

Tipping and Donating


Like sending money in the mail, it is possible to offer monetary support without providing your identity or address. Additionally, with Bitcoin the receiving party doesn't have to provide its personal information.
The low transaction fees make it economically feasible to send smaller amounts than by mail. This is especially true if the receiving party is located in another country. It's easy to send just a few cents to another internet user to commend him for an insightful contribution.


Buying digital goods


Buying access to a newspaper article, a movie, or an mp3-download currently requires you to create an account with the merchant and, often, to create a balance first. Bitcoin could allow such purchases without exchange of personal information for a global market.
Especially for small purchases, it would be likely that merchants would offer a discount, as receiving bitcoin payments is significantly cheaper, easier to implement, and less hassle, since they can't be charged back. On the other hand, any sufficient reputable service would not be interested in scamming the customers for cents, potentially alienating its userbase.
For example, I could easily see Bitcoin become a much used payment method for computer game vendors such as Steam or GOG, as it already has become for HumbleBundle.


Online Gambling, Pornography, and Drugs


While we might not like it, Bitcoin as a payment method has advantages for services of questionable morality and legality, so there are some that operate solely on Bitcoin or at least offer it as an alternative payment method.
It might be preferable to users to pay by Bitcoin, than to have a pornsite's charge on the credit card, while pornsites have a lot of trouble with chargebacks due to buyer's remorse and for other reasons.


Bitcoin is a fast and cheap way to send money to a private person abroad


Definitely faster than an international wire, or even TransferWise.
Likely cheaper than Western Union or PayPal (depending on exchange courses and amounts).


You don't need a bank account


For a lot of people it is significantly easier to gain access to Bitcoin (just internet access), than to get a credit card or even a bank account. Currently, not having access to banking is a major hurdle in order to participate in online markets.


Travelling


Far down the road, it might be easier to carry just Bitcoin while traveling than to exchange to the local currency for each country. Already, one can pay taxis, hotel rooms, air fare, and food delivery in Bitcoin.