See:

https://www.google.co.uk/search?ei=blR4WuCiIIWhsgHk6qiQBA&q=...

and

https://www.google.co.uk/search?q="loss years" accounting&ei...

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Note added at 49 mins (2018-02-05 13:19:34 GMT)
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Also see:

https://www.insurancebusinessmag.com/uk/news/breaking-news/2...

Nov 22, 2013 - A S64 ITA 2007 claim cannot be made as Mike has no income for the loss year 2009-10 nor, because of the claim made for the 2008-09 loss, for the preceding year. Profit of the trade in 2007-08 is nil, for the purposes of the FA 2009 claim in respect of the 2009-10 loss, because it was extinguished by the ...
https://www.gov.uk/hmrc-internal-manuals/business-income-man...

It made a chargeable gain in that year of £10,000. It had made a total profit of £50,000 in the previous 12-month accounting period which ending 31 December 2012. It can set its loss against the chargeable gain in the year of the loss (year ending 31 December 2013) so that it pays no Corporation Tax in respect of that gain.
https://nelsonsonline.secureclient.co.uk/nelsons/index.cfm?e...

Oct 31, 2017 - It may be that the taxpayer's income of the year before making the loss exceeds the amount of loss relief available whereas the income of the loss year may be much lower. In these circumstances, it is likely the taxpayer would explore electing to treat the losses to be set first against his previous year's ...
https://www.taxation.co.uk/Articles/2017/10/31/337160/reader...