Hi, everyone thank-you for all your balanced responses.
I see both sides of this and genuinely understand everyone's points, especially when it comes to online booking systems and data protection. It most probably depends on the culture of the organisation, and that's why everyone's responses are different.
Just for a bit of an update, as a coincidence, my husband said today (like 30 minutes after posting this thread) that he wanted to check what holidays he has left the rest of the year and to book more in. He pulled up this myhr website in front of me and put in (already saved on the laptop) password and the username, and it showed his holiday entitlement, what he has taken and what is left and when is holiday are.
I asked him some subtle questions, and it turns out if he asks for holidays online on this website, he finds out a response by going back and looking at it, and the work does not send an automatic email there were a few days inputted by HR for Christmas and New Year. He also said he needs two weeks notice for A/L.
My husband's position is not a high level one where he runs specific projects and tasks, and he does not manage anyone or is responsible for things that others do not pick up when he is not there. There are like say four people in his role, and they all have the same level of authority. I also said that my husband is starting masters soon.
By the time he needs the time off (Saturday and Sunday), his contractual days will be those days. I could just tell him that we are having a family meal on a Saturday night, can you request those days off work please and he would probably be none the wiser but my only fear with this is that what if he asks or gets asked to do overtime the following week (from the Monday onwards). We leave a Sunday and return the Thursday in the afternoon.
By this time of the year, his exams will have passed, and his dissertation will not start again until the next couple of weeks after the holiday so I do worry about overtime coming up and how I get around that.
I could ask him to get the Saturday/Sunday as A/L and then once that's approved I could call his HR department and tell them about the surprise trip and to kindly make sure he is not given overtime for the remaining part of that week.
If I did tell him to take annual leave for a whole week, he would get suspicious. We are also a very traditional couple. We tell each other everything, and there will most likely not be anything else he wants to use that A/L for, at most he does not use it all and forgets to take time off.
FYI, I am not springing the trip on it at the airport. I plan to tell him the day before over a nice meal and give him some money to go and buy himself some clothes. The holiday is for Barcelona, and it's only for 4 nights.