Seriously, if you can't afford a big wedding, you don't have one. Or maybe you get lucky and your friends and family rally round and do a potluck for you - I would be fine with that, but it doesn't seem like that is the case here. If you simply don't want to pay for a big wedding, I don't think you have any business asking your guests to foot the bill.
I do believe that people should have the wedding they want, not the one their guests want, but they should be sensitive to their guests' needs, and shouldn't expect that everyone will come. For example: "It is our dream to have our wedding in a barn, and we think it will be a lot of fun to camp out for the weekend. However, we understand if you are not comfortable with camping and can only make it to the day portion. There are hotels located x miles away."
About the cash gifts: well, if your guests are paying for your wedding, you should strongly suggest that you don't want any gifts, it's just cheeky otherwise. In general, I'm fine with registries and have seen both UK and US weddings mention them either on the invitation or on the website. But, almost everyone says "we understand that attending a wedding is expensive and your presence is already enough of a gift for us. However, if you still wish to buy a gift then...". I really don't have a problem with giving money for a honeymoon either. Honestly, it's a luxury that they don't really need whether it's that or a pasta machine so what's the difference?
Overall I think what is totally lacking in this wedding is any appreciation for the guests. I have flown over the pond a few times for weddings now, and people have always been very appreciative of the time and money that we have put into it, and I'm glad they do because we do have to make sacrifices in order to attend. But last year I went to a wedding here in the US where I spent a ton of money on things like the bridesmaid dress, bachelorette party and raffle donations for the Jack and Jill. I certainly felt like acknowledgement for the money I had put into it was lacking along the way (although I did get a thank you card afterwards). It would have been nice to hear "I really appreciate you spending the money on this" or, "don't feel like you have to bring something if you can't afford it".
I also had a birthday the day after my SIL's wedding last year. It was totally forgotten by my in laws (even though we had talked about how close it was to the wedding many times). People just get caught up in themselves on the day. You could get your DH to mention it to the couple and say he is bringing along a cake? Or just write it off and celebrate it another time. Or could you leave midday Sunday and celebrate with DH? I don't see why you should be obliged to stay until Monday.