It is not necessary to claim the maximum capital allowances available or even claim them at all, crazy as it might sound there are situations when not claiming capital allowances can reduce your tax bill!
Sole Trader Example
The personal tax allowance is currently £10,600 (2015/16)
Lets assume profits are £15,000 and Capital Allowances available are £5,000, so that would reduce taxable profits to £10,000 which would waste £600 of the personal tax allowance.
It would therefore be better to only claim £4,400 in capital allowances and claim the remaining £600 in the following year.
Companies within a Group can only offset losses in corresponding tax periods, so if the the capital allowances increase the loss in one part of the group beyond the profits of the rest of the group then there would be no benefit to claiming them in that period.
Companies can claim capital allowances in any of the following 3 tax years.
There is an excellent example of this in the following blog http://taxnotes.co.uk/a-basic-introduction-to-capital-allowances/