In addition to considering the student loan repayments - which are a serious burden - you do need to consider Income tax + National Insurance (which was much lower) to compare like for like.
When basic rate tax was 33% in 1978:
Personal allowance (if married): £1535 (£8437 current value after inflation)
Income tax at 25% first £750 income, ie from £1535 to £2285 (£8437 - £12560 current value after inflation)
Basic rate 33% next £8000 income £2285 to £10285 (£12560 - £56533 current value after inflation)
NI 6.5% between £17.5 and £120 per week and nothing above that cap (£5000 - £34300 current annual value after inflation). Now NI is 8% between £12570 and £50270 and 2% above that
2025
So someone on £45k in 2025 pays £64686 tax and £2494 NI so has a take home of £35,919.60
If they have a student loan to pay back, that comes down to £34,115
1978
So someone on £8190 in 1987 (£45k current value after inflation) would have paid £2136 tax (£11740 equivalent) and £346.45 NI (£1901 equvalent) so has a take home of £5707 (£31,369 equivalent)
Difference
So despite the big difference in headline rate - 20% vs 33% income tax tates, our sneaky governments have added all sorts of hidden extras and layers so the actual take home pay isn't much different: £34,115 versus £31,369 take home pay
I think it would be interesting to do the calculation for £22k (full time NMW) and for eg £70k income and see how that has changed too, including benefits. I believe that redistribution has increased significantly - but it will take some googling to check what benefits there were then (and now!)