Yes of course she is shocked , embarrassed and humiliated . Id be worried too about the money that people have spent.
But she needs to call it off today . The usual way of doing this is that the hosts ( people whose name is on the invitation ) contact all the guests with this one line
“ The wedding of Claire and Alfie that was planned for Saturday 26 August will no longer take place “.
Normally that’s done by post but in view of the timescale here, email would be better . Someone else (NOT claire , her parents / bridesmaids etc ) needs to phone anyone for whom you don’t have an email address.
You don’t give anymore explanation in writing that that. People who need to know the detail will phone Claire’s mother / best mates etc . You need to protect Claire from having to deal with a hundred phone calls.
That gives guests time to cancel their hotel booking ( most will give refunds until a few days beforehand ) and return new dresses / outfits. If there is anyone from her side that she knows is in a dire financial situation because of this, Claire can always offer to give then some money towards it .
Id not recommend this but it it might make Claire feel better about it and it will cost less that £150,000. Most people who have a £300k deposit also have friends who can cope with the loss of a hotel room deposit.
Most employers will let staff cancel annual leave with a few weeks notice . Or people can make other plans for the day.
She needs to do this today . Guest will be annoyed if she knew it was off but didn’t let them know asap.
Freinds / bridesmaids / her family also need to hep her notify all wedding suppliers IN WRITING asap and check the contracts to see how much money they can get back . Also notify her wedding insurers ( if she took out insurance ).
Alfie almost certainly is cheating on her . I know two people who were in this situation . The first bride cancelled - all the guests were shocked but understood . None of us wanted her to marry that man.
The second bride didn’t - she persuaded him it was just cold feet and to go ahead . The marriage lasted 4 months and ended in a very tragic way.
I’m very sorry for Claire but better a broken engagement than a divorce. This is going to be a few weeks of work and a few months of humiliation. A bad marriage will be years of pain and hurt for her and her child. let alone the financial consequences which Pp have explained .