Our Self Employed package doesn't have a place to enter goodwill. This is because even the long version of the Self Employment submission for HMRC doesn't include intangible assets. Another reason for this is that a business run under Self Employment isn't a distinct entity from the proprietor so the funds to purchase the takeaway would be yours personally and you benefit from the realisation of that good will turning into profit over time.
I expect you are looking for a way to count the goodwill aspects of the purchase against tax over a number of years... As far as I know the allowances for "Goodwill written off" only apply to Corporation Tax, so you'd need to set up a limited company, seed this with £30K share capital and buy the takeaway business.
The assets purchased would be fixed assets which would have their purchase value be written off against tax. It may help to look again at the value placed on the assets incase there is room for adjustment.