Self-Employed Expenses (Computer Hardware)?

I do software (app) development on a freelance basis. In order to make iPhone apps, you must use a Mac, which I don't have. I've been borrowing one but really need to purchase my own.

My question is, can I count a Mac computer as an expense, as it's the only way I can do the work?

So for a job I have lined up, can I deduct the cost of a Mac as an expense (and therefore reduce the taxable net profit?)

Depends, will that supposed business asset be used for anything else? If so, deducting the expense from your income gets really tricky.

Your computer would be classed as a fixed assets rather than a general expense, the revenue however will allow you to claim capital allowances on this which will be offset against your tax liability.
This may also come under the scope of your annual investment allowance so it may be allowable within the first year of purchase.