Copyright and Patents Act
Devices to circumvent copy protection - "chipping" (includes internal & external chipping devices, eg. chips & cartridges)
S.296 (1) Applies where a copyright work is issued to the public by or with a licence of the copyright owner, in an electronic form which is copy-protected. (2) The person issuing copies to the public has rights against the person, who knowing or having reason to believe that it will be used to make infringing copies -
a) makes, imports, sells or lets for hire, offers or exposes for sale or hire, or advertises for sale or hire, any device or means designed or adapted to circumvent the copy protection, or
B publishes information to enable or assist persons to circumvent the copy protection
Quote is direct and specific. In the UK fitting modchips is illegal, even supplying help on fitting and info is in breach. Be aware of that the guy from London who thinks the Roms and chips with cromwell are OK
To the original poster legality depends on what and where as you can see by the example above.