Self employment expenses

I have a client whose only income is from self-employment, they haven't filed their tax return for last 2 years. Income was about £5k per annum with £3k expenses, so net profit of £2k per annum.

There will be no tax pay due to personal allowances but client is asking if they can simply declare the £5k income with no expenses, effectively carry the expenses forwards and offset it against any future years when there may eventually be a taxable profit?

There are genuine expenses but client doesn't want to waste their personal allowances, what are peoples thoughts on this? I can restrict depreciation in the tax comp but how about other expenses?