VAT can be reclaimed on Farm buildings and fixed equipment, but as a computer is not fixed permanently in a building then you cannot reclaim the vat on it. You may be able to claim capital allowances on a portion of the cost of the computer, ie if 20% of the time you use the computer is for farm accounts, then you should be able to write off 20% of the cost of the computer over 8 years.
For a computer costing 1,000, then you could claim capital allowances on 200 over 8 years, ie at 25 per year. It is not a lot, but every little helps.