TCGA92/S56 (1) provides that the apportionment of the cost on a part-disposal is to be made before the indexation allowance is computed, see CG17352.
Since 2008 indexation allowance applies only for the purposes of corporation tax. (CG17207).
Restriction of losses: capital allowances – TCGA92/S41
In the case of an asset subject to capital allowances, the apportionment under CG12731 should be made first and capital allowances taken into account only to the extent that they relate to the part being disposed of.