Before end of term my ds school emailed to say any child off sick before the end of term I. E the Thursday or Friday or the start of the new term will need doctors proof as they see a rise in absence tagging onto school holidays
Although some do lie about it, some schools seem not to grasp that it really is possible to get sick on any day, regardless of how close it may or may not be to a holiday.
There's also the consideration that a lot of schools wind down in the last day or two before the end of term with fun activities rather than actual learning. What's the point in relentlessly pursuing parents and/or giving kids a black mark for attendance just because they weren't there to watch Toy Story and play snakes & ladders?
I agree with the PP - if GP surgeries are going to do this on the whim of a school, then the school should be billed for the GP's private service. There may also be a case for the parent claiming back travel and parking costs to and from the GP too.
Going to a GP and wasting precious NHS time is like getting the garage to send out a recovery truck and give your car a full diagnostic to see why it's stopped working when you know the fuel tank is completely empty and you could have just walked to the petrol station with a fuel canister.
What is the GP actually going to prove anyway? Anybody can put on a chesty cough, talk like they have a cold and ham up the 'feeling sick' symptoms. At any rate, you could just lie about having been sick or had diarrhoea. Are they going to berate you for not grabbing your phone in time and filming in real time inside the toilet pan as whatever nastiness emerges from either/both ends? Or expect you to film the results afterwards - which could easily be mocked up with gravy or soup anyway, if you're that desperate? How dignified 
There's also the fact that feeling terribly sick with a bug or whatever means you just want (need) to stay in bed for the day (or two) until it passes. If you know you're going to have to get the child up and go out anyway (and possibly be on the phone all morning trying to get a last-minute available appointment), you might figure that you're just as well to get them up and send them to school. Once they're at school, they aren't going to learn anything feeling like that and may well be disruptive - all they'll do is pass it on to everybody else. Then the school is left with 30 potential absences and having to generate and follow up 30 lots of 'we assume you're lying unless you prove otherwise' letters.
Incidentally, schools sometimes phone parents during the day to come and fetch their child if the child has been sick or otherwise taken a bad turn. How would they like it if a busy working parent demanded that they email 'proof' that the child really is sick before agreeing to come and collect them?
It's so sad that it's come to this - and I know it's pressure from the government and LEAs driving it. When I was a child, parents had a good working relationship with the school, based on mutual trust and respect. We could request up to two weeks of holiday in term time (my sister and I usually had one week) and this was granted, unless there were concerns about a child's attendance or other circumstances. Children who were constantly absent without explanation were followed up, but there was never any automatic assumption that the parents were lying if a child was off sick for a day or two - they just asked for the child to take a brief note from their parents when they returned (mainly to ensure that the parent was aware that the child had not been in school), filed it away and that was that.