I’ve merged a lot of mine – just did it myself, very simple – fill in a form with the new company and they request the fund from the old provider and it gets transferred for you.
My experience is that in the past it was easy, now (and for the last few years) it is a nightmare. I’ve been trying to move one (only worked there 15 months so either I get it transferred or they send my a cheque) for the past year.
The laws mean that the one you move it too has to prove that they are offering the same/better benefits, and it’s not as easy as it sounds.
And tbh, if you don’t have to transfer it, don’t. Eggs and baskets.