Being a vet and a doctor are two very different things, there are a lot of cross overs, but ultimately they will involve different things and lead to different career paths, there are a few fundamental differences between the careers but they often seem to be lumped together as being similar.
You will need to decide within a couple of years which route you want to take, whilst the GCSEs you are doing/a-levels you chose will probably be the same for both courses, like you say work experience requirements for vet med are high and different to med, so you need to decide where to spend your time.
At the moment you probably are best trying to spend some time in both areas - finding work exp at a dr's will probably be difficult, but try for any healthcare setting. Ultimately when you are applying for vet med you will need experience in vets, farms (dairy, lambing), kennels, stables etc...so time in any of those sort of places would be good.
Obviously one of the main areas that is different will be your patients - vets treat animals (but also have to deal with people, so not a option if you don't like people, as some people seem to think!) and doctors treat humans. So which can you see yourself wanting to treat/work with? Some people prefer the fact there seems to be more 'worth' in treating people to animals, other people couldn't deal with operating on a person etc. Are you going to get frustrated that there are options that are technically available, but you can't offer them to an animal patient as they aren't available for animals yet, or the costs are too much - whereas for a human you can do as much as possible. Equally though, medicine has to deal with the NHS and the issues that can bring with it. Lots of possible +ves/-ves to think about what you would be happier working with. Personally I am happy dealing with people (farmers/pet owners), but would not want to operate on or deal with sick/ill people.
Vets tend to end up being more of a jack of all trades. yes, there is chance to work in one field and increasingly more and more vets are specialising. But when a vet qualifies they are a consultant/GP, surgeon, give health advice, nutrition advice dentists, podiatrists, oncologist, radiographer and so on. Whereas doctors will end up more limited in the areas they work in, but are very skilled in that area. Again, which would you prefer?
There are other differences as well, but it is really down to where you can see yourself being. Try and arrange a week with a vet and a week in a healthcare setting. Hopefully that will help you lean towards one or the other. What sort of placements can you see yourself doing? A week on a farm covered in cow poo, trod on by numerous cows, working outside in the cold, dark and pouring rain, chasing sheep around a field, up 3/4 times during a night in a cold sheep barn up to your elbows in a sheep with a rotten, stinking lamb, cleaning up a kennel of dog diarrhoea at a vets etc. Or in an old peoples home and all that entails, bed bathing people, dealing with feeding tubes, people dying, helping people go to the bathroom etc. Most people will realise pretty soon which career they would be ok doing - and which they wouldn't!
One other point I would say as well...bear in mind there is a huge differences in salaries. If money is something that would make your mind up (esp if you compare it to what your brother will earn as a doctor). Even at the highest earning point of their career vets will be earning a lot, lot less than a mediocre doctor.