Allen has already made a big impression since joining the professional ranks from Millfield School in the summer of 2005.

He made his first team debut in the 2005-06 season playing in Gloucester’s 34–6 victory against Bristol in the Anglo-Welsh Cup. In the same season, he played a pivotal role in Gloucester’s triumphant Middlesex Sevens squad in 2005, scoring two tries in the final against London Wasps. 

After breaking into the senior team at the backend of 2005-06, he began the 2006-07 season brilliantly and forced himself into the England reckoning, picking up two caps in the defeats to New Zealand and Argentina in the Autumn internationals. He is yet to add to that tally. 

Capable of reading the game superbly, Allen is one of the new breed of youngsters forming the backbone of Dean Ryan's side. He played a key part in Gloucester’s run to the quarter finals of the Heineken Cup and the play-off final defeat to Leicester during the season of 2007-2008, contributing with some fine tries and some outstanding defence.

With the arrival of Olly Barkley from Bath, Allen faced stiff competition for the number 12 jersey in 2008-9.

However, he went on to make 16 appearances before opting to join Leicester Tigers for 2009-10 when his contract expired at the end of the season.