As mentioned, there is road fuel duty to add (it was 6ppl last time I paid it, but it's a lot more now), plus you should be charged VAT at 20% instead of (IIRC) 8% for heating fuel. That's going to eat into your savings.
Now, you'll need a tank with bottom takeoff (not all of them have it) and a pump. You'll need a "proper" setup, and the gas vendor may wish to see an inspection certificate for the installation - the minimum I've been asked for is a tank certificate. You can pretty well forget most of the pumps for sale on eTat - they just aren't suitable.
Very, very occasionally a used Smiths LPG pump will pop up - and from memory they've gone for "good money" approaching four figures
A proper hand pump (like the Krug one I was lucky enough to pick up) are rarer still. I really hate to think what a new one will cost.
Just now and then a complete setup will appear - tank, plump, hoses, etc on a skid. Again these tend to fetch good money.
As an alternative to buying your tank, most of the gas vendors will rent you one - but that ties you into only using their gas supplies which prevents any shopping around. You'll be tied into a contract, but after the initial tie-in you now have the legal right to switch and have ownership of the tank transferred to another supplier. This bit came in a few years ago because suppliers were locking in customers who didn't want the upheaval of having one tank taken out and replaced with another.
Buying your own tank means you can switch suppliers and in theory negotiate on price based on what their competitors will supply you for. In practice, I've never been able to get the major vendors to quote - or if forced quote a price that's clearly designed to get rid of me. Anecdotally I've heard this isn't uncommon - eg Shell will only fill a Shell tank, and so on. So owning your own tank may well mean no-one will supply you.
And then there's the metering - two aspects to this.
Firstly, if you already have gas for heating, contrary to popular belief you CAN use the tank for both heating and vehicle fuel. However, HMRC won't let you buy heating fuel and use it as road fuel. You can either buy all the gas as road fuel, meter the heating gas, and claim back the difference in duty and VAT; or you can buy it as heating fuel, meter the road fuel, and pay the difference. I believe a vapour meter is a LOT cheaper than a liquid meter (and for tax purposes, the taxman will want something "proper"), though I don't have any prices for either.
Secondly, if you get together a group to share a tank, then you'll want to know who's used what - and that means metering the liquid although since it's only for your own use it doesn't need to be a high accuracy one.
On tank siting, from memory the restrictions are roughly :
For smaller tanks, less than 500l, it's 3m; for larger tanks it's 4m. Thats from the tank (and I assume fittings) to any buildings, boundaries, unprotected electrics and open drains. The location must also be such that the driver can observe the tank while operating the pumping gear on the tanker. Those limits make it impractical for most people - though when I've discussed it with potential suppliers in the past I've had the impression that there's
some flexibility.
Taking all the above into account, I more or less gave up on the idea a long time ago.