I was signed off work with mental health problems at the beginning of February. It was a pretty rough time, but I'm back to my old self now and doing good, and I was able to return to work this week 
I was on full pay for the first month I was off, and then dropped to statutory. Frankly, I'm grateful to have received anything, though I think we can all appreciate/imagine how tight things can get financially while on SSP.
Anyway, for the past three months I have pretty much scraped by. I haven't fallen behind with anything and all of my payments for rent/bills, etc. are on track. We've had enough left over to keep me, 9yo DD, the cat and the dog fed and happy. All good. I haven't gotten into any debt to manage a standard of living we can ill afford right now, and I hacked our food budget to the absolute limit in May to treat DD on her birthday and pay for her school trips (they do a 'fun' week every May at her school and take the children on various outdoor pursuits. The thought of going cap in hand to the school unable to pay this mortified me, so I hacked every budget I could to pay it).
A good friend of mine got married earlier today. DD and I received an invitation to the evening do back in April. I have been putting whatever spare pennies I had in a jar to buy DD a pretty dress for it since then. Today we were able to go out and buy her a beautiful dress that we found on a sale rack and a cheap pair of sandals - they were total bargains and I still cannot believe our luck in finding them. I had no money for an outfit so I'm recycling a dress and a pair of shoes I found in my wardrobe - I'm hoping beautiful hair and makeup will carry me through it as the outfit isn't great 
My problem is a wedding gift/money. By the time we received the invite in April, the budget was eaten up with other things, including putting Holy Communion money aside for a friends set of twins. May's budget was dominated by DD's birthday and school trips. I got my SSP yesterday but council tax, childcare and various other first of the month type bills have demolished the lot. There's a little left over which will buy us food until my tax credits are paid in 2 weeks time. I've got nothing to give my friend for a gift. I've bought her a beautiful card, but I've got no money to put in it (they've got a wedding post box for cards/vouchers, etc.). Another close friend text me earlier to ask how much I was giving, and I explained the situation. She text me back: You can't give them nothing! It's their wedding! How the fuck would you feel if they did that to you?!
I can't give what I don't have. I could certainly pop some vouchers/cash in the post to them in two weeks time when I've got a bit of money, but I have nothing right now. Is that horribly unreasonable of me? I feel dreadful about it, but I can't change it. It is what it is. I was hardly going to prioritise a wedding gift over my DD's simple birthday this year or ban her from the school trips which gave her a very memorable week learning lots of different skills and experiences.