I had children at MTS, NLCS and Habs all at once !!
Order of the rest.... I don't know the St Albans preps very well at all.
If they have a junior bit Aldenham and royal masonic for the ahem... academically challenged =- nothing wrong of course with schools where just about everyone gets in and they had add huge value. I went to a tiny girls' private school in the NE and did really well, top of the class etc. It can work for some children.
My habs daughter who is slightly dyslexic but very clever and got in at 4 a year young in fact in those days nearly didn't pass to the seniors (she did pass and thrived when handwriting and spelling was not as important and now is City lawyer on about £100k by the way and happily married...) At 11+_ she sat for Habs, NLCS (did not get in there), and then second tier? st Helen's, third tier - Northwood College and almost a boarding school but we heard she had passed for Habs before the entrance exam for that. She sat for six but that's 5 so I've obviously forgotten one.
If it's a prep school without seniors as it may be for a boy look at where the boys go on to - is it the better schools or not and decide on that basis. Also check how many scholarships are won at 13+ - 3 of my sons won music scholarships at 13+ when changing schools. (Two are still in school so I don't write about their school).
Do remember that most children sitting for Habs will be sitting for NLCS etc too so it might look like huge numbers per place but if you do a bit of maths you can see that some will apply to a lot. Also depends on the competition. Our second who was the only child of ours who read very well and very uoung they could hardly find a book she couldn't read at the Habns interview where her sister was at 4+ and didn't get in (triplets got in although I am not blaming those girls). So she went to Kensington prep for 2 years in London and got into NLCS at 7+ - 18 and is also a London lawyer now. To be fair their MTS graduate brother is currently a postman so this is not a show off post.
I do think ti helps to read to them every night, talk to children a lot, ours all learned 2 music instruments from age 6 or 7 ( we are very musical family) and all the usual stuff. I thin they are 50% genes and 50% parental effort and of course hard work - amazing how much hard work makes you do well in life not something everyone wants to hear.