The Inland Revenue ceased to exist several years ago on amalgamation with HM Customs and Excise to form HMRC. One HMRC official of whom you probably have not come across is the one originally known as the District Valuer who'es main task was assessments for rates and Council Tax. If necessary that official can assess a current market value.
You are slightly correct with your 18 month rule. I should perhaps have mentioned this. In your last 18 months of ownership you are indeed deemed to have been in residence even if this is not the case. Take your total ownership in months and deduct 18. Then the latter figure over the former is applied to the gain to reduce the gain exposed tom CGT slightly. However, as you never lived in this house this concession almost certainly would not be allowed.