I think the problem here is that essentially what you are doing is an extremely selfish thing, but you want a way to come out of it without looking selfish.
I think that the poster above has nailed it.
I know you really like the idea of getting married in Mexico, OP, and I'm sure you have some very personal reasons for it being so important to you, but a wedding is a family and social occasion, which you obviously agree with, as you are inviting 100 people, but by asking 100 people to come to MEXICO you are pretty much being the worst hostess you possibly could be, by putting 100 of your dearest friends and family into a difficult and inconvenient position. They don't want to go to Mexico. If they did want to go to Mexico, they would be doing it under their own steam. You are asking them all to spend thousands of pounds coming to Mexico to watch you get married. So let's just say a conservative estimate of £1000 per head, you are asking your friend and family to spend £100k on your wedding. Do you see how that seems selfish?
If you love your friends and family and want them to be there to celebrate your marriage with you, then you will have a wedding that they can actually attend and enjoy without anxiety or hardship.
If Mexico is important to you as a couple, which I'm sure it is, you will then have a wonderful, stressfree honeymoon there afterwards.
You're reading this and thinking "I SAID that I honestly am not bothered if nobody comes!" but really? If I get an invitation tomorrow in the post to your wedding, are you really saying you don't give a damn whether or not I come? Then why the hell are you inviting me?
Oh, because you love me and want me there, right? Okay, I'll come. So now I have to worry about whether or not DH and I can afford it and what we're going to do with the kids for a week, or am I really going to fly to MEXICO for 48 hours because I can't leave the kids with my parents for longer? And now I've also lost a week of my annual leave. I didn't really want to go to Mexico anyway. :/
Yeah, I don't think you actually love me or care about me at all, do you? Because if you loved me and wanted me (and the other 99 members of your guest list) at your wedding you'd have your wedding close to home.
(You did come on IABU, okay?)