Allen first worked as a nightclub disc jockey where he met Dale Winton and subsequently joined the United Biscuits Network, a radio station broadcasting to workers in the United Biscuits factories. His voice was heard in Scotland, Liverpool, Manchester and London.
UBN closed in 1979 (following the lorry drivers' strike during the Winter of Discontent) and he joined LBC as presenter of the Night Extra programme. The Night is Young was the next series of programmes he presented and when LBC was separated into 1152 AM and 97.3 FM, Allen presented the afternoon show of 'London Talkback Radio' (latterly LBC News 1152) and Saturday Night Out. He has also worked for Blue Danube Radio in Vienna, a station sponsored by the United Nations. He was also the host of Crinkley Bottom radio on Noel's House Party.
His current radio programmes are the early breakfast show between 4am and 7am Monday to Friday and between 5am and 7am on Sunday. In Conversation, his second show for LBC, is broadcast on Saturday at 6 am and Sunday at 9 pm and features interviews with guests from the world of entertainment. Highlights include Julie Andrews, Lenny Henry, Victoria Wood, Michael Flatley, Andy Nyman, Paul Daniels, Dale Winton and Paul Zerdin.
In 2006 and 2007, he embarked on a live tour visiting theatres around the London area. All dates sold out within a day of going on sale.
In 2009 and 2010, Allen continued his sell out 'audience with' stage shows at the Queen's Theatre in Hornchurch and at the Mermaid Theatre in Central London. He was supported by fellow LBC presenter Anthony Davis.
In February 2011, he performed again at the Mermaid Theatre, supported by mind-reader Graham P Jolley.
His rapid banter takes in news topics as well as incidents and irritations from his daily life. He is known for his critical comments on minor celebrities including Cheryl Cole, Katie Price and Sarah Harding.
His fellow LBC presenter has been known to be on the receiving end of Allen's comments. In a handover from former The Apprentice contestant James Max, after being introduced by Max, Allen said "Oh go away, you boring old windbag!". The following week Max was criticising a minor celebrity while reading the newspapers. During the handover Allen said "It's no good nicking MY act!".
Allen was one of five presenters of 5's Company. a magazine programme broadcast on Channel 5 when it was launched in 1997.
He has been diagnosed as diabetic and has since promoted testing on and off air.