It's more like the Model 3 is cannibalising S/X sales (particularly the S) combined with the fact that the S/X were overpriced. They were making crazy margins on those cars before, especially the P100D's.
They also made some changes to the auto pilot options, regular auto pilot now only comes with auto steer (like AP1). All of the 'enhanced AP' features are now bundled into full self driving package meaning you now really need to get both.
You are still looking at the best part of £100k after adding paint, wheels and full AP for a P100D and £110k for a 6 seat X (including the £3.5k grant).
No free supercharging and the 4G (inc satellite maps, live traffic and music streaming) in the car is now a subscription (1st year included).
EDIT: That has to be a pricing error on the UK site, the US site doesn't include ludicrous mode in the base performance price and it is a $15k option.
On the US site a fully loaded Model S P100DL with 21" wheels, red paint and full auto pilot costs $129,750 (no taxes), that's £98,240 + VAT - £3.5k incentive. I put that at £114k all in.
On the UK site the same spec car is £99k all in.
It seems we are getting the ludicrous speed for free currently. $15,000 is about £11,360 + VAT (£13,632). Differences in exchange rate explains the last few % away.