As both Shine On Me and Moonstomp are set mainly in north east London, and as it's my tipple of choice, lager top gets served in several of the pub scenes. It's a drink particularly known in London and less so round the rest of the country. 
Simply it's a pint of lager with a splash of lemonade, about half an inch, sploshed in when the lager has finished being poured. A shandy is an 'alf and 'alf drink; the top is just a dash.
Lager became popular in Britain after the 1958 film Ice Cold In Alex made it look particularly refreshing. It's still something of a Southern drink but mass marketing and pub chains have stamped all over interesting regional differences in beers. 
Time was lager was of varying quality from pub to pub and so the dash of lemonade made drinks palatable even if the beer wasn't necessarily. 
American 'zine Boots 'n' Booze wrote approvingly and Santa Cruz embraced the refinement. During the early 80s Lord Wastrel writing in Sounds music paper was carousing with the punk, reggae, metal, and even futurist bands of the day and would often imbibe, occasionally with disastrous effect, and make mention.
Don't be forgettin' that a gold watch along with yer beer rounds an evening off nicely.