Hi Jane, I'm assuming this is for a sole trader? If so yes you can set off trading losses against general income of the same year (there is a 12 month time limit from 31st January I believe, may need to double check this though). Normally I would say a bird in the hand etc so claim this but if it is expected that there is going to be dramatic turn of events and profits will be made next year that would take your client into the higher rate band for tax then it would probably be worth carrying forward.
Further thoughts on this Jane after refreshing my memory is that you would have to use the whole loss and therefore waste the personal allowance of £10,500 and only get relief on the marginal £795 thus getting a refund of £159. Unless there is unlikely to be future profits I would carry forward.
Another quick question losses - this has been helpful this post - if a sole trader is using the new cash basis are they allowed to carry forward losses? on the gov.uk website it says something about not being able to carry forward - I may have misread it
