Mobile broadband is available from all the standard mobile networks. If you want it here and now, it may be best to go in to a shop and purchase a mobile broadband dongle. If you are not in a rush, you can browse the offers and deals available from the mobile networks on their websites. Click below to check out what is currently available from each network.
Note that following the merger of Orange and T-Mobile into Everything Everywhere, it's recommend to purchase direct from EE to take advantage of their 4G network rather than through T-Mobile or Orange directly.
Remember, that mobile broadband signal will vary from place to place. You can test how well a network will perform by comparing the signal using a mobile phone from that network as these uses the same technology as mobile broadband dongles.