Returning to flying is much easier than some who have posted information above might suggest.
First of all, decide what / where you wish to fly. If you're content with just VFR in UK airspace, you merely need to complete refresher flight training as necessary, then pass a Proficiency Check for a Single Engine Piston Class Rating and submit a Pilot Medical Declaration. This will give you PIC privileges on aircraft of up to 2000 kg with no more than 4 PoB. You may also fly Microlight aeroplanes using that licence, provided that you receive conversion differences training from a Microlight FI. But the format of your UK PPL is so old that it would need to be re-issued, so you'd be better off converting it to a Part-FCL LAPL at the same time - then you'd be pretty well future proof.
If you wish to extend those privileges to anywhere in an EASA Member State, as a minimum you will need a LAPL Medical Certificate and a Part-FCL LAPL. If you wish to fly something heavier than 2000 kg or with more than 4 PoB, you would need to convert your UK PPL to a Part-FCL PPL, requiring you to obtain a Class II medical certificate.
None of the above requires you to take any theoretical knowledge exams!
If you wish to fly under IFR, you will need an instrument flying qualification. This cannot be included on a LAPL, so you would need to convert your PPL to a Part-FCL PPL before adding an IR(Restricted) which is identical to the IMC rating - but is only valid in UK airspace.
Any reputable training organisation should know all this.