The Westfield XI is going to be much more like a series 1 Lotus Seven than any Caterham, especially one built in the last 20 years. With a period engine (BMC 948 - 1272, Ford 1172 flat head, Ford non-cross flow or Climax if you want an engine that is worth more than the rest of the car) it will be 1100 lbs and rocking 50-90 HP. Its better aerodynamics will let it reach 100 mph and run the quarter mile in about 20 seconds. Skinny tires will give less grip than an MGB.
If Westfield hasn't corrected the problems of clubman XI's (straight axle) or series 1 Sevens it will:
- Have a significantly better turning circle to the left than the right
- Grind the rear U-joint against the frame on hard turns in at least on direction
- Have an exceedingly narrow footwell & tiny pedals
- Have scary drum brakes
Since many of the parts are interchangeable with a Lotus XI there it is unlikely all the idiosyncrasies were addressed.
On the other hand it will break away gently and lurid slides will be easily recoverable. If you want style and classic driving get the XI. If you want performance go for a modern Se7en.
I admit that I have never driven an XI. I do have significant time in a series 1 Seven.