It's difficult to see, but is it only thin triple wall polycarb? Our last house conservatory had 5 layer 25mm and it was much warmer than we expected. You can get 25mm or even 35mm polycarb, so might be worth investigating if that upgrade alone would be enough
If the roofs not leaking, surely 100mm celotex between those rafters and some internal pvc cladding under gets the biggest bang for your buck? Few hundred quid and a weekend of easyish DIY.
This will give instant results while you save up/consider your next move. The expensive bit (celotex) will almost certainly be reusable if you decide to further upgrade.
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