An office in your garden – even if it is movable – counts as property and not plant and machinery. So you cannot claim Capital Allowances (and reduce Corporation Tax or Income Tax) on the cost of the building itself.
However, you could potentially claim Capital Allowances on some of the internal fittings and furniture in the office. Also, if they are separately identifiable on invoices, you can include the integral features of the building – such as the electrical systems or water system – in a Special Rate pool and Claim Capital Allowances on those at 6%.
And, if your business is VAT registered, you may be able to claim any VAT back on the cost of cabin – but this should be restricted only to the business use proportion of the cabin.