blondes with the greatest of respect, glibly asking whether someone who can't have a baby is going to do surrogacy - when you admit you don't understand the ins and outs of how it works - is much like someone glibly asking whether you've thought about adoption.
I am in London. Surrogacy in the UK is basically impossible unless you have a friend or family member. Commercial surrogacy arrangements are to all intents and purposes illegal - so there are very few altruistic surrogates willing to carry a stranger's baby. All three UK surrogacy agencies are closed to new intended parents, because the ones they have on their books are never likely to get a match. The legal system also doesn't support surrogacy as there's no way to enforce any agreement. So a surrogate is under no obligation to give up the baby for adoption to the intended (genetic) parents. Although rare, there are horror stories like this
http://www.nataliegambleassociates.co.uk/blog/2016/10/30/uk-surrogate-vetoes-legal-parenthood-for-biological-parents-even-though-she-has-no-wish-to-be-involved-in-the-lives-of-the-twins-she-carried/
Unless you have a friend or family member willing to carry for you, surrogacy in the UK is not an option.
So that's why couples have to go to the US for surrogacy.
In the US commercial surrogacy arrangements are legal and there is a legal framework. Meaning that in surrogacy friendly states not only are surrogacy contracts legally enforceable (ie they can't change their mind and keep the baby), but you can get a pre birth order whereby your names go straight on the birth certificate. Although the UK still doesn't recognise these, you're at least able to have a birth certificate without the surrogate and their spouse's name on it
The surrogacy agencies in the US recruit and screen very carefully, so you know no one is being exploited. You pay for them to provide counselling services and ongoing liason with the surrogate and their family. You have ongoing contact with them and can fly over for scans and the like, and obvs you are there for the birth and take the baby home. The framework just isn't there in the UK to protect either the surrogate or the intended parents
The alternative is the very shady world of surrogacy in eastern Europe - specifically Georgia and the Ukraine. Where women are paid peanuts and you aren't allowed to have any direct contact with them, or even meet them until after the 12 week scan. Women who are desperate to feed their families are rented for wombs, and you have to trust that someone you've never met or spoken to will keep your baby safe for 9 months. You can't go to antenatal appointments - someone just grows your baby and you get the odd email and scan pic.
£100,000 is probably a conservative estimate for surrogacy in the US. Depending on the state you could be looking at more like $150,000. You have the cost of the surrogate's fees, agency fees, screening fees, US legal fees, health insurance for the surrogate and for the baby, IVF costs, plus all your travel and accommodation.
In the event of a more complicated pregnancy you have additional expenses like lost wages for additional bed rest above and beyond what's planned for a normal pregnancy and delivery
And that doesn't include UK legal costs for applying to the high Court for a parental order and getting a British passport
So no, surrogacy isn't just putting one of our embryos into someone else's tummy for love and expenses. It is a great deal more complicated than that
You are correct that surrogates in the UK don't get paid - but because of that, it's broadly impossible to access.
And regardless of where you do it, the biggest challenge is emotional.