I've got a Halfords HB063 41 AH. The size is 8w x 6.75d x 7h inches (203 x 170 x 178 mm)
I got it because the original failed and I had to get something which would fit the car from the a small local shop. It's the "wrong" one, but does the job fine. Never had any steering issues or strange behaviour other than a period of disuse during the lockdown. A quick charge and all was well.
The "correct" battery is Yuasa HSB012 Silver. £103 from Halfords or £59.35 on eBay.
I dont believe the car needs a huge battery. The 14AH battery in my 1200cc twin cylinder motorbike did the job fine for years. Two 600cc high compression pistons are far harder on the starter than even the four 350cc slugs on the 100HP. The bike battery was replaced with a 4AH LiFePO4. Yes that small. It worked perfectly.
The Panda has a big alternator that shoves out useful power at low engine revs. IMO, if you don't leave the lights on with the engine off there's no reason why a 40AH or less should not continue for a very long time.