Many years ago there existed the UK Government Powershift scheme which did make a contribution towards the cost of adding LPG equipment to conventional petrol powered cars. I am not sure if any such scheme is still in existence. I do know that anything the gov't paid was subject to tax as part of your earned income.
Are you really sure you want to run on LPG? There are issues associated with it, including a reduction on power and torque compared to petrol operation. Clearly with a 4.6 litre engine losing 15% or so of available power and torque is not likely to be a serious handicap but there are also [unverified by me] reports of valve seat recession (wear).
This could amount to nothing more than the typical British attitude which is along the 'know what I like & like what I know' kind of lines. In much the same way as [most] people didn't want to accept unleaded petrol, didn't want to accept diesel. Their current phobia is electric propulsion.
This largely British phenomenon is possibly responsible for Islamophobia and general racism here in the UK.
If you do go the LPG route I would suggest careful research, and definitely don't buy the low end of the market. Sequential LPG injection is the way to go in my opinion. Simple evaporators placed in the intake tract are crude and in my estimation, plain dangerous. Consider factors such as integration with your car's onboard electronics. You don't want spurious warning lights glowing at you during LPG operation. Well, I don't anyway.
Keep me informed of your thoughts and progress!