I don't have experience of LPG but I can tell you about oil prices. When we moved into a 200+ yo house in July 2021, oil was 45p a litre. Then Russia invaded Ukraine and come the beginning of March 2022, it was £1.30 a litre. There are 10.36kwh per litre of oil.
This year it was 85p a litre when we moved again in July (exceptionally high for a summer oil price) and is now down to 59p a litre.
The price is extremely volatile, and there are no government caps.
We got an extra £200 from the Government as an alternative oil payment in MAY (!!).
We're currently considering buying a house that's on oil but with three rooms (and a cooker) on gas; it only has a 650 litre oil tank, which terrifies me - firstly because the minimum order with almost all oil companies is 500 litres so we would be forced to be on some sort of top-up system (which costs 10% more) or constantly be monitoring oil and risk running out in cold spells. You can have a 'watchman' installed so you don't have to use a dipstick; in the rural location of our last house, the signals never worked, so it was all manual.
Even now, in our current home with a 1300 litre tank, we can't easily take advantage of waiting for the summer price to drop to its lowest to fill it for cheap over winter. We'd absolutely have to get a refill come January when prices are at their highest.
Really, if I had to take my pick of houses on oil, I would want a 2,500 litre tank so I'm not at the mercy of wildly fluctuating prices.
You should really get an oil heating engineer to talk you through because I don't really have a clue what I'm talking about, but I reckon the bigger the oil tank, the bigger the boiler needs to be (bigger oil tanks will mostly be installed for bigger homes, which definitely do need bigger boilers).
The only thing I know about LPG is our friends had a hell of a time finding any for some time last year, and it's gone from £60 to £93 a bottle.
Regulations may have changed since your current oil tank installation, e.g. needing a 30cm solid base the whole way around didn't exist in previous years:
https://www.oftec.org/consumers/off-gas-grid-guides/home-guide-to-domestic-liquid-fuel-storage-up-to-3500-litres