If you are an existing Orange user looking to get the best deal on the market for your iPad then you can get a 500MB monthly package from France-telecom owned company for a mere £5 per month.
The package comes with a few strings attached though; you mustn't fear to use a knife and a scissor to convert the SIM into a microSIM and you must agree to stay with Orange for 18 month in all.
Orange's terms and conditions document states that "Your mobile broadband SIM is for data usage and text messaging from your laptop only, and in particular, does not include voice calls. If you remove your SIM and use it for any other purpose, you will be charged for that service at standard network rates."
Since you're actually using it only for data purposes, embedded in the iPad, which is essentially a laptop without a keyboard, Orange shouldn't penalise you; after all, Orange can't verify whether or not you're using the dongle provided.
Orange's soon to happen merger with T-Mobile almost guarantees that the Joint Venture will have the best network coverage in the country. Be careful though as Orange charges for out of bundle usage, 2p per MB on 18-month contracts.