M4 style stock only fit the G3 with an adapter. There is still the issue of the length of the adapter. You add a few inches more onto the G3 because of the design, and a pre-made M4 kit might have tubes that are a little short.
LCT bodies use a slightly different shape to the real steel G3. If you fit a real steel wood kit the real steel adapter needs to be swapped to the LCT one.
You can modify the steel body to accept real steel back plates, but then the LCT ones don't fit correctly.
Or you could buy the LCT that already has the adapter installed, So you know it fits, and know you can buy other LCT stocks in the future and swap between without any issues.
The LCT LC-3AR AEG is the only model that comes with this adapter and stock pre-installed. I've not seen one for sale in the UK yet but HKTiger111 have them for sale.
https://airsoft.tiger111hk.com/p37350/LCT-LC-3AR-AEG-Rifle-(Black)/product_info.html
That model doesn't look as good as the standard G3 to me. And It's already a heavy gun in the standard format, so the addition of that RIS, and the Loss of the stock weight may cause the rifle to be very unwieldy.
The other option is buying a Standard G3 and the adapter plate. You would still need to swap the end caps but the advantage there is the end cap will be a direct fit with no modification.
There are still no guarantees the M4 kit will have long enough tubes for the Airstock, Or that it will look good when done. So You may end up having to search for airline extensions and fittings. And that depends on where the reg is placed. If it's on the bottle standard 10bar straight connectors will be fine, If it's on the gearbox you would need extended pipework. I don't know that system to comment.
https://airsoft.tiger111hk.com/m310/LCT-airsoft/p37169/LCT-LC-3-series-AEG-AR-Stock-Tube-(Black)/product_info.html