Find the web page you refer to but their 'small print' on the Dell Finance Page includes this:
'Finance from Dell Financial Services (CIT Group (UK) Limited). UK consumers aged 18 to 70 years only, subject to status. Prices shown may vary without notice.Administration fee payable in the event of settlement of loan at the end of the Holiday Period. APR dependent on credit score and may differ from typical rate shown. Terms and conditions and written quotations available upon request. Credit Card deposit required.'
If you give a link to the page it might help to check whether there is any 'small print' on Terms and Conditions.
I do know how annoying it is when advertised offers actually have 'exclusions' in them, but I would guess that for them to supply expensive equipment with no charge whatever, they would need a long and (as far as possible) blemish free credit history.
I don't begin to understand what they were saying about not continuing with the order affecting her credit rating!