Let’s say your current business has been having a tough time and you want to change it to something new, can you carry forward the trading losses.
Probably not look at this example from BIM85050
For example, a publican who had owned a pub in Leeds for many years sold it and bought another in York. Although in the everyday sense the trader remains a publican throughout, the York pub is not the same trade as the Leeds pub.
Tax law requires any losses (including Corporation Tax Losses) carried forward to be offset against future trading profits from the same trade.
One solution to this may be Group Relief, companies which are part of the same Group can surrender losses within the Group.
The rules about which trading losses and other amounts may be surrendered are described at CTM80110. The company that transfers the losses, etc, is called the ‘surrendering company’. The company that claims the losses, etc, is called the ‘claimant company’.
Trading losses, excess capital allowances and non-trading deficits on loan relationships may be surrendered in full. This is irrespective of whether the surrendering company has other profits against which the loss etc might have been, but has not been, set off.
Alternatively it may be possible for the loss making business to sell services to the new business and in doing so reduce its loss.
steve@bicknells.net
Business Accountant 
Reblogged this on Mike Tombs's Blog and commented:
We’ve had a couple of enquiries recently from people changing trades and hoping to use the losses from the old business against the profits from the new one. The rules are quite strict where losses from previous years are carried forward – in general they can only be used against the old trade. There are some useful ways of using losses in the early years of sole-trader businesses to claim back personal tax paid in previous years.