Anna, I would imagine that someone tutoring for the 7+/8+ would not be tutoring for Common Entrance as they are VERY different exams.
As some have said, the tutoring necessary for the 7+/8+ does not require someone withe advanced degrees or even someone with teaching qualifications as they are looking for potential at this point, not necessarily attainment. My son's pre-prep only have a quarter to a third of the year (sometimes less) going for the 7+/8+ thus do not spend all of year 2/3 prepping for exams yet they are successful having only an exam club which meets twice a week after school for one term.
What is important is that your sons have covered the curriculum, are familiar with the test style questions, and have had practice with timed exams. It is these things that a good pre-prep will ensure your sons have. If your sons will not be at a pre-prep, you can do this yourself at home easily:
English - have your sons read a wide variety of texts. Colet Court would have sent you a reading list you can use for level expected. Ask your sons a variety of reading comprehension questions, bearing in mind that what is expected at 8 is different from what is expected at 7. Use Bond books at and 1 year above age level for written practice. At 7, they must be able to answer the question with a complete sentence (using the wording of the question within the answer). At 8, they expect and extension of the question in the answer (because..., therefore....).
Writing - your children should be able to conceive of a story and write something with a clear beginning, middle, end in 25 minutes. THis is just practice. It helps to give your children a list of 5-6 "Wow" words that they practice using in the stories.
Maths - It seems that your son is already very good with mental arithmetic, so just make sure he is familiar with the topics. Practice some exams. Not much to do if they are good in maths.
Verbal/non-verbal reasoning. - practice with bond books
That is it! Not very difficult to do. Competition to get into the very selective schools is actually more difficult than the numbers given to you, however it is due to parents pushing children forward for the exams who probably are not suitable. I would really seriously think about the schools you wish to apply to and pick those schools which suit your child - the list you gave have schools with very different atmospheres and where different kinds of boys thrive. The exam is just one day, they will have to spend many years at the school - hopefully happy, but would be miserable if there is a mismatch. You mentioned that your sons are sporty and outdoorsy and that Colet would be your first choice - I would then urge you to look at Kings College (no affiliation, and my son is not at that school) as they have similar feelings. Kings is also much more sensible in it's application process at 7 and 8 as it interviews all boys and invites all boys to an Activity Morning to observe boys as they are - these 2 factors bear heavily in the decision to accept or reject the candidate.