"Where a car has been extensively modified or converted, certain Reasons for Rejection such as 'components missing where fitted as standard' should not be applied, for example: A car converted for rally use (i.e. rear seats removed and fitted with a roll cage and full harness seatbelts etc) may have been converted so as not to require a brake servo, power steering or airbags. VOSA says it is expected that modifications to cars converted for track or rally use will be permanent and readily identifiable, and wouldn't include those where an airbag has been disconnected for "occasional" off-road use, for instance. We're also told fixed drivers' seats are acceptable in any vehicle provided the adjusting mechanism has been removed and the seat is now clearly a fixed seat in one position. Simply removing the adjuster lever or locking it in one position won't be acceptable for an MOT.
http://www.pistonheads.com/gassing/topic.asp?t=1130476