I know a 'normal' roster is a thing of fantasy, but I assume following a long haul 3-5 trip you would get around 4 days off. So by deduction you should get approx 3x 4days off a month with 3-4 trips.
The rosters don't work like that. You MAY get 3-5 day trips with 4 days off afterwards but the network is so variable (flights from 25 mins up to 17 hours) that you usually get a mixture of long and short trips with 2-3 days off, occasionally up 5-6 days off... (all based on the 777 rosters - the 330/340 is mostly turnarounds and 2 day trips).
As such would it be possible to commute to LHR on a regular basis?
Almost certainly not unfortunately. Especially as flights are often full and even if there are free seats you get bumped off for cargo payload (on standby tickets).
Also WRT pay, I have worked out that take-home pay would be around £5000 a month plus accomodation allowance of £2300? (Assuming I do not take up the company accomodation and get a flat in town for around £1000 a month).
Also I have 3 children in the UK can I use the education allowance on UK schools and if so how much to they pay?
Yes, with certain limitations - you need to read the policy on education allowance comprehensively as it's quite complex (think it's found on the emiratesgroupcareers.com website).
Anyone also know Time to command it seems to be around 3 years it this still the case?
At the moment guys who joined 3 years ago are upgrading on the 777. Airbus guys have waited about 4.5 - 5 years due to slower expansion. Hard to predict how it will work out in the future but I'd plan on 5-6 years on the 777 and a bit longer on the Airbus if the A350 doesn't come as fast or in as big numbers as originally expected (likely to drop the A350-1000 for the 777NG).
Hope that doesn't put you off... Despite it's faults it's still one of the best packages out there when you factor in the quicker command than comparable legacy carriers.