Transferring to the UK - NI vs SocialSec/MC

I'm working with my company on a case to move me from our US office to the UK and wanted to get a good view on what that would cost to maintain.

Company Cost:
Salary - $80K
US Medical - $19.2K/yr (fully paid by company for family of 4)
Social Security / Medicare @ 6.2%/1.45% - $6.1K

Assumptions:
If I am there for <5 years I will be able to claim exemption from NI, but still have to pay US Social Security.  Myself and my family will be covered by the NHS in this scenario.
I will be paying much more in taxes in the UK compared to the US due to having a family of 4 and the UK not having a married/child exemption.  Therefore I will not owe any US income tax.
My wife doesn't work and won't be for at least 2 years (a discussion for later when the kids go to school if we are still there).

Questions:
1. If I claim NI exemption, how much will I pay in Social Security/Medicare taxes?  The full employer+employee 12.4%/2.9%?
2. Will my employer be exempt from their portion my NI for my salary as well, making the fully loaded cost of my employment mostly my salary?
3. When will I pay SS/MC taxes?  When I file my US return in one payment?
4. I live in a low cost of housing (but not necessarily living) area of the US.  I expect a jump from $750/mo to $2000-3000/mo going to a location within 30-45 minutes of Central London.  Yearly train/tube tickets for me and my wife look to balance with car insurance/gas so that isn't a worry.  Anything else to consider as far as cost of living increase in pay?
5. If my wife were to seek part time employment she would not be restricted from doing so if I were on a tier 2 visa?

If you elect to be covered under the US-UK Totalization Agreement, both you and your employer will pay US Social Security and Medicare taxes at the usual rates on your UK salary. Alternatively, you and your employer can both pay UK National Insurance contributions instead. In either case, your employer must take the applicable social security taxes out of each paycheck, just like now. This would only change if you were self employed.

On balance you will probably find the UK (particularly the southeast) more expensive than the US, particularly if you are coming from a relatively low cost location in the US, but some aspects of living in the UK can work out much less expensive. The single biggest whack is probably the higher rates of UK income tax, although this is offset by usually having no state income tax to pay.

If your wife is a Tier 2 dependant, she can work. In fact her entitlement to work will be considerably more generous than yours, since you must work for your Tier 2 sponsor but she can work for any employer.

The totalization agreement is not elective; it is mandatory on the relevant parties. You remain within US social security and have the relevant contributions deducted or paid by the employer.

It is quite easy to confuse this with the US/UK tax treaty; which is elective; so only applies if one elects for the benefits it may provide.

The totalization agreement is not elective; it is mandatory on the relevant parties. You remain within US social security and have the relevant contributions deducted or paid by the employer.

It is quite easy to confuse this with the US/UK tax treaty; which is elective; so only applies if one elects for the benefits it may provide.

True enough; but there are actions within the control of the OP and his employer that affect how this all plays out. For instance, if the OP's employer sent him for longer than five years he'd be covered under the UK system. This would apply even if it was just on paper and they decided midway through to bring him back after less than five years. Similarly if the employer has a UK affiliate and the employee becomes employed by the affiliate this could result in having an option to be covered under the UK system.

I cannot see why they would want to do this since it would cost them more! Just making the point that the OP and his employer can elect to structure his transfer in certain ways that result in being covered under one system or the other.