I hope you do not mind my replying, not as a pupil or parent but as a teacher of several years standing. Of course, I am biased and it is probable that some have a different experience of the school. With that proviso...
How strict is the school?
KEGS is a fairly liberal school and much is placed on self-discipline. It has some 'zero-tolerance' rules which attract cynicism from the pupils (e.g. uniform, running against a red man on the dual carriageway) but for the most part the school seeks to run in a collegial way. The higher up the school the pupils go the more oppresive they find it, but I guess that's just school in general.
Regimental? like being in the army?
That actually raised a smile. We (staff) often joke that instead of an 11+ we should just line up all the boys and give them five simple instructions to be completed in sequence. Anyone that could do it is in. We'd have about 10 boys in the school. Even the CCF struggles to be regimental at times!
Do boys get called by their first or surname?
First name these days, unless I just don't know about it.
Do teachers constantly shout or talk to the boys as grown ups?
The teachers don't really need to shout so they don't (as a rule). My experience is that all are treated in an adult way but that is perhaps down to perception. I don't hear a lot of shouting as I go around school.
Do the boys feel they enjoy school or do they feel pressurised to achieve at all times? is there room for error?
I think a lot of boys, without pressure to achieve, simply sink into apathy. Almost all boys choose to come back in the sixth form and are devastated if they do not get the grades. Those few (one or two per year) who choose to go elsewhere usually are going for a particular course, such as the IB. Very few dislike the school and many love it. Again, my perception.
How much homework do they get in the early years?
Year 7 is a lot. It's a shock for everyone. After that it depends upon the boy. Some parents complain they have too much, others too little.
Can boys use their ipad/android and mobile phones?
At the discretion of the teacher, they can use smartphones and they can use the school's BYOD (bring your own device) network for the internet. This is just starting out and many teachers are a little wary at this early stage. Expect use to increase. I haven't seen any iPads (except the teachers, more space for pupil humour there!) and I would be wary of trusting an 11 year old with such valuable piece of tech. Seeing how they leave their stuff everywhere I wouldn't recommend it!
How ethnically diverse is the school?
It depends on the year but in year 7 it is about 50/50 White British/BME
Which sport(s) do KEGS excel in? Where do boys go swimming?
Rugby and Football are the main sports and happen on Saturday mornings through the Autumn and Spring terms. Cricket is popular but doesn't happen on a Saturday. All three sports regularly have teams in county finals. There are Rugby and Cricket tours (Italy and the Caribbean). There are other sports as well with regular fixtures but you would have to ask someone more knowledgeable.
Are the boys allowed to choose which A-Levels/GCSE to study, regardless of teacher assessments? I know some GS dont allow their students to take subjects if their teacher doesn't think they will achieve an A grade
For A level there is generally a GCSE grade requirement to take it to A level. At GCSE there is free choice, no restrictions as far as I know.
What is the school dinner like?
Well the portions are big. Jamie Oliver lets us have chips once a week (Friday) and that is (surprise!) the most popular day of the week. There's a published menu for hot food, there's a pasta bar, sandwiches, salads (usually just the teachers of course), fruit and cakes.
I hope that has helped. Feel free to PM with any other questions.