Vodafone will unlock the handset for £20 with no upfront payment required (only Orange demand this). If you are over 6 months into a Vodafone contract they will usually unlock the handset for free but this depends on your service provider; some are better than others.
T-Mobile handsets are always locked but some retailers use generic, unlocked "GSM" stock if they've run out of T-Mobile provided handsets. Carphone Warehouse are a prime example.
Don't forget that the 6600 is a DCT4 handset and as such it will probably be possible to unlock it using the simple software floating around on the internet. You can have this done for £1 at the CrazyNokia.co.uk forums or for $USD5 at a number of US based sites. Codes are returned both on-site or via email and are pretty much instant.