It would be prudent to refrain from depositing for this offer until they can be bothered to provide a minimum level of CS.
A blank page for the terms and conditions would not comply with UK laws regarding how such terms are displayed to the consumer, but rather than get into an argument after you have won and they claim terms are violated, send this offer to the ASA pointing out how it violates the required standards (it's an advert after all, and the ASA have included internet advertising in their remit for a while). The CMA might like a copy too for information, as it's an example of the kinds of sloppy marketing and presentation of terms that leads consumers to end up potentially being treated unfairly.
In theory, displaying a blank page for the terms and conditions might mean that a court strikes out the whole lot as at the time they were not available to the customer, which itself makes them unfair.
An example of this is private parking companies. They make money by charging £100 for parking not in accordance with the terms, but they are losing cases they take to court where the defence can show that at the time of parking, the terms were not clearly displayed to the driver on entry, or not sufficiently lit such that they could be read by the driver entering at night.
In this case, you would probably find there was really no 100% bonus either, and then they would try to charge you for trying to get your money back without playing. If there was a 100% bonus, it would also be on their website, which would provide an alternative route to the terms and conditions.
This is most likely a rogue affiliate using spam, and advertising an offer that does not even exist as "click bait". This would be where the ASA comes in, as in law the casino is held responsible for any advertising contracted out to third parties, and that includes affiliates. Affiliates continue to behave like this because they are allowed to get away with it, leaving players to suffer the consequences of misleading offers by finding their is no bonus after they have put their money in, or that the offer text contradicts the terms, such as an offer that promotes Roulette as the click bait, but leads to an offer that is slots only, and where play on anything else leads to winnings being confiscated.