The NIR 450 class trains known as the castle class trains for Northern Ireland, should they have been sold to England?

That is a bizarre proposition. The NIR class 450 stock was built in the late 1980's and was not really new then, as it was built using salvaged underframes from old BR Mk1 coaches and refurbished engines and traction motors taken from scrapped NIR class 70 stock. Hence a lot of the parts of the trains actually dated back to the 1960's. 

Leaving aside the issue of Northern Ireland having a different gauge from England, the idea of 'wealthy people' being attracted to 'luxury' journeys on what amounts to worn out outer suburban multiple units seems unlikely, even if the stock was regauged and allowed to run on the West Coast Mainline (which it would not be as it would cause lots of delays due to being a lot slower than modern trains).

There is only ONE 'Castle Class'. they were the Great Western Steam locomotives.

They carried names like 'Clun Castle', Pendennis Castle', Drysllwyn Castle', and in railway terms they were 'fair old performers'!!!!

No, as they would require extensive modification to work properly on the UK mainland... the main one being the UK mainland uses standard gauge (4ft 8.5-inch) rails, while in Northern Ireland they use a broader gauge 1600mm (5ft 3-inch) track.

No. There is little spare capicity on the line from Manchester to London Euston as there are already 3 trains per hour doing the trip in 2 hours 5 minutes. For those requiring luxury there is a first class section with complimentary food and drink

That would not work the lines are very busy no room for any more services.

The Class 450 was a type of diesel multiple unit (DMU) formerly used by Northern Ireland ... They were affectionately nicknamed 'Thumpers' and 'Castles' (Also known ... trains were constructed by British Rail Engineering Limited between 1985 ... On Thursday 9 March 1995, a 450 Class train was used to form a royal train ...