I think whether they improve or not does depend a bit form subject to subject. So someone getting a C in English now is largely showing C skills . They'll learn more texts but their ability to write essays won't change (much) alth0ugh in some subjects NEA has an impact.
In other subjects they tend to make more linear progress - the sciences and maths.
But UCAS grades, whatever, should be on the optimistic side. So, if teacher thinks A/B, UCAS should be an A.
This is what did cause such enormous backlash with a (national) group of parents after CAGs as they did expect their DCs to get their UCAS grades!
There is also a lot of bargaining at this time of year where a student 'needs' an A because they have found a course they want to do that asks for AAB and their teachers think they are BBB- BBC. That's always a tricky one. Especially if the student decides simply having the A predicted means they will get it!!