I don't understand why you think an internally installed timer would be more likely to result in the machine being on and unattended than an externally fitted timer or are you saying it would be illegal either way to switch the machine on in the morning to warm up if it was before people arrived? Please bear with me, I'm not a complete moron, I'm just not familiar with the UK regulations.
Don't think your a moron at all fella, how it's switched is irrelevant the problem may POSSIBLY be with thing firing up at all while there is no one in the premises, no issue with configuration of timers at all