As Globaliser noted, if you use any cookie or ad blocking tools, it isn't likely to track - so use an 'unprotected' browser for it.

One other point you might not be aware of: I did this using the 8 Avios per pound Apple promo last year, and it is on the price before tax (and postage where applicable - I picked up in store). Given VAT is 20%, on a £799 iPhone you actually ended up with the bonus on ~£560 of spending. Still good, and even better at 12 per pound, but just a warning in case it is less than you expect. It is in the T&Cs.

I can't remember if it applies to other retailers too - I suspect so, but am sure others could confirm.