If you're travelling to Croatia from the UK then yes, you need to present a negative PCR test (done in the previous 72 hours before travel) or rapid antigen test (done in the previous 48 hours before travel) to enter Croatia. There are numerous companies offering these types of tests "for travel" (i.e. they produce a proper certificate with your details and details of the test) - your airline may be able to offer you a special deal on them, or you can (likely) even do a rapid antigen test at the airport.
Looking at the UK gov's guidelines a UK citizen needs to provide a PCR certificate but this is expensive, but the antigen one isn't so clear which is free or cheaper. I can see you can get a certificate to show you have had an antigen test done but I can see a whole load of people just turning up with a test done themselves with the home test kits.
It does say you cant get tested at the airport on arrival.
What are people doing?
Are they really turning people away if they dont have an antigen certificate.
To visit Croatia from the UK you need a negative PCR test taken in the previous 72 hours OR a negative rapid antigen test taken in the previous 48 hours. In the latter case, the test needs to provide a proper certificate with details about the traveller and full details of the test (name of test, name of centre, manufacturer, time and date taken, result and so on). You cannot use the NHS tests for travel, if this is what you mean. I believe the airlines in the UK will ask for the test certificate as well before you board. You can pre-book a rapid antigen test through somewhere like Boots, or get it done at the airport before travel, or some travellers have said their airline offer special test deals.
When you ask "are they turning people away...", who do you mean, the Croatian authorities when you enter? Well, if you don't show a negative test certificate you can still enter but you will need to self-isolate immediately in Croatia and test yourself there, being able to leave self-isolate once you get a negative test.
It would be far easier to get a test in the UK before travel!
Was in Croatia in July then you could not board the plane back to the UK unless you had pre booked tests in the UK and filed a location form at gov.com 72h before arrival the test centre in Pula is very good p.c.r results in 6hrs that was Amber but now green so it will be different