"It is illegal under international agreed and locally applied leglislation to modify a Ham radio
to transmit on all chanels IF"
IN the US, under ITU and FCC regulation, no.
A ham radio
may not be commercially sold unless it meet type acceptance, and does not transmit out of the ham bands, but that regulation applies to manufacturers and dealers, NOT HAM LICENSEES. A ham radio
licensee in the US is personally and completely responsible for the legal operation of their radio
(s) and allowed to make any and all modifcaitons that they please, including building equipment of ay type from scratch.
If you doubt that, contact our FCC, there's no debate.
As a ham radio operator you are also allowed broad liberty with "emergency operation". And for that reason I have legally
modified my ham VHF radio so that it is capable of operating in the marine VHF bands and other frequencies, so that if I have to abandon ship and the normal marine VHF is lost or non-functional, I can legally use the ham radio, on marine band, to place a perfectly legal distress call.
Such operation is in fact encouraged by our regulations, the amateur radio service is allowed to occupy valuable airspace for several reasons and public service / emergency operations is probably at the top of the list these days.
Your rules in the UK probably will vary, but in the US this is legal and proper. By federal regulation, for emergency operation, pretty much anything goes if it will accomplish the goal of saving lives and property. WIthout causing excess mayhem, obviously.