You have already answered your own question although you haven't accepted the answer. You need to reread what you wrote, particularly one part of it:
He eventually left for a holiday with another girl and I was stranded on my own, not knowing what to do. I contacted an ex boyfriend who was still a very good friend and he offered to help me. I planned on keeping the baby and he wanted to be there to take care of us (incredible, I know).
He "eventually" went off with someone else. Eventually?
Now, put this matter in perspective:
About 13 weeks into the pregnancy I lost the baby.
So, here was a man you
viewed as your soul mate . . . again, soul mate is the term you use . . . who in less than three months, was sufficiently involved with another woman to go on vacation with her.
He did not eventually
go happily off on holiday with another woman. Eventually is not a word that fits in this time span. You miscarried at "about 13 weeks," so, in less than three months, he impregnated one woman . . . and, yes, it does sound like an accident . . . and went off with another on vacation. Frankly, it sounds like he was either seeing both of you at the same time, or, that he is a constant troller for women, or, that he went off with someone of very recent acquaintance to spite you/salve his own feelings in re: this accidental pregnancy/both.
Your use of the word eventually shows that you are rationalizing his bad behavior.
You need to stop doing that.
I can understand that you are vulnerable. You married a man when you were perhaps 23, the age at which women's hormones make sex most appealing and necessary. Two years have gone by and your marriage remains unconsummated.
I think an annulment is in order. You use British expressions but that does not mean you live in England. I have no idea where you are or what the laws there are but an unconsummated marriage between people your age (assuming your husband is an age peer) is not a marriage unless there is some unique physical reason why there is no sex.
Yes, that will leave you emotionally unsupported but the chief problem in your life has been men you can not depend upon.
As for Mr. Holiday with Another Girl, forget him. If you can move, move. Do not leave a forwarding address, phone number or internet address. You have been wise not to see him in person . . . but please think hard about a man who disappeared with another woman while you were pregnant, who has not seen you in four years but who now wants to "start a new life with him."
While it is true that you are no longer the person who became pregnant four years ago and he may have grown up, what he is suggesting is a fairy tale.