If your car is truly non compliant, then you looking at replacing the headlights (point at a wall, at night, the high part in the US is on the other side). If you look at the wall, the top of the beams are like this:
Yours: _______ _______
US: _______/ _______/
You’ll have to check at your local dealer to see if the emissions on the car will meet US specs. In recent years, manufacturers have gotten pretty good at making motors, and make one for all markets. It used to be that motors were different for different markets/countries, but the difference is slowly fading. They must be willing to state - in writing, on Porsche letterhead - that the car meets US specs. You also have to look at bumpers, and maybe other lights. Glass has different requirements here as well. Seatbelts, exhaust (catalytic converters must be current), and the list goes on. I can’t list everything, and the guys around me came up with a couple more.
Right now, the going price for a 10 year old Boxster S is right about 13K. Only your research will tell you what it’s worth in England.
I was going to bring back a Mini Pickup in 2000 (they’re kinda rare), but the cost of import was crazy. They were telling me it would cost ~10K to modify it. I sold it there instead. You have to have the modifications done there, too, as customs won’t release it in unless the US spec’s are met. You’re not allowed (in 2000, anyway) to take the vehicle to someone who can fix it.
So…you can define expensive.