This would be standard incompetence.
This happens as the "left hand" sends out the mailer offers, but the "right hand" of the organisation is responsible for crediting them. In this case, the "left hand" never bothered to pass the fact this offer was made over to the "right hand" who manage the accounts. Hence, CS see no "note" on the account and cannot credit the bonus (they have no authority to countermand the rule - no note, no bonus).
Eventually, such activity will lose them customers. Why has this not happened in this case?
For a UK player, the choice is wide, there are many places that offer better bonuses, and for a small deposit amount such as 45, 100% should be possible for a regular player. This might be monthly, or even more frequently.
King Solomon has a UK Pound version, although it is less well known than the dollar version. Bonus coupons are frequent until you reach a lifetime positive balance, not something the average player will manage.
32Red will give 50 per month, and BelleRock have a regular monthly of 75 specifically for UK players only (their CS leaves a little to be desired though).
If you must have the English Harbour experience, uninstall the software, and wait for the inevitable mail asking if/why you have lost interest in the casino (they know damn well how much competition there is in the market). When it comes, tell them why, including the total amount of bonus funds you were offered, but were denied once you had deposited, and why you have thus lost interest in all the subsequent offers they sent you.