For your A-Levels I'd take Economics, History and a language. The grades don't matter all that much as long as you get into a decent university.
At university I'd take a language combined with either politics, economics, philosophy or history. You'd need to get a 2:1 as your final grade.
I would recommend doing a master's degree after that.
I would make sure you try and do as many internships as you can during uni. Try getting them abroad with international organisations, NGOs or think tanks.
Following this, thee are two routes you can take if you want to become a diplomat: 1) Career Diplomat and 2) Freelance
to become a career diplomat, you need to take the Civil Service Faststream entry tests ((http://faststream.civilservice.gov.uk/)
which all professional grade government employees need to pass. This will require in the first stages taking tests on numeric and verbal reasoning, alongside in-tray exercises. Should you be successful here you'll then be invited for further testing. These will require group tests and they'll be watching your character. They look particularly for people with the ability to reconcile different opinions, not the loudest talker or the fastest to solve a problem. Then you'll need to start on the Foreign Office track of testing. This is a difficult path to go down and competition is fierce. Should you be successful, you'll be invited for intensive interviews with several different panels. You will also have to undergo medical examinations.
Get through all that (an extremely difficult task) and you will have a career for life as a diplomat. You won't neccessarily get to chose which country you go to either (particularly when younger). As a junior, you are most likely going to be sent to some far-out embassy in Sub-Saharan Africa or Central Asia. As you get older you'll be working in places of greater interest to the UK. The key places to work here are Washington, Brussels, Paris and Beijing.
To go freelance, you'll need at least a Master's degree in a relevant subject (internatinal relations is usually the best). You'll need to do internships to generate professional experience first. Once you've got your master's you will probably have to do at least two six-month internships. Try getting them abroad with organisations such as the European Commission, European Council, the Council of Europe, Organisation for Security and Cooperation in Europe (OSCE - an excellent organisation to do internships with due to the variety of places in which they operate), the United Nations, or one of its agencies (a good one here is the United Nations Development Programme). These are all excellent diplomatic organisations to work for after your internships.
you will need to keep your eyes peeled for jobs at entry level constantly. keep track on sites such as unjobs.org, reliefweb.org, devnet, and eurobrussels. Most entry level jobs require at lest two to three years professional experience (which you can begin to gain through internships), but occasionally one pops up that doesn't require as much experience.
Once you secure your first job, it will usually be for 5-7 year contracts, so you keep having to look for new jobs but the benefit here is that you can decide where you want to work. There are generally two types of contract here: seconded, or contracted. A seconded job is where you are employed by the Foreign Office on a temporary basis to represent the UK in an international organisation. These are hard to get, and you will need to get hold of the desk officer for secondments within the FCO to back you for the position. The other way is to go for a contracted job. These are easier to get but the seconded jobs are usually filled before the contracted ones, so there are less of them.
Don't believe too much in the 'it's not what you know it's who you know' adage. On your journey, make sure you are reliable, professional, friendly and try to go out for drinks with colleagues when you can. don't be too pushy when trying to build your network. believe in yourself remain optimistic. amke sure you read the FT and the Economist as much as you can and keep plugged in to world affairs in your spare time.