Yes, they have every right to charge you.
In every bank's terms and conditions booklet (which they have to give to you according to FSA regulations, and which are freely available in PDF on the 'net) they explain that they can charge you a fee for going overdrawn over the agreed limit, in addition to the % they'll charge you anyway.
By signing the application form, you are agreeing to be charged if you go over the agreed limit.
Certain banks like Lloyds TSB will waive the first fee you incur for going overdrawn, but I'm not sure what Barclay's policy is for this...