The transfer of business assets by an individual to a company is a disposal for capital gains tax purposes. The disposal proceeds are deemed to be market value because the sole trader and the company are 'connected persons'. See the Basic calculation principles of capital gains tax guidance note.
The sole trader will therefore have a capital gain on the chargeable assets at the point of incorporation. The chargeable assets will usually be land and buildings and goodwill. It is unlikely that gains will arise on other assets such as plant and machinery as these will either be standing at a loss (for which relief is given via the capital allowances computation) or any gains will be exempt under the rules for wasting chattels. See the Chattels guidance note.
The trader can defer the capital gains on chargeable assets by claiming either incorporation relief or business asset gift relief. Any gain that is not deferred under these provisions may be reduced by claiming entrepreneurs' relief.