Reasonable expectation would include:
- Tax equalization. Meaning that you are not liable to 2 tax systems. You should continue to pay the US tax only and your company should pay all UK tax. All the extra benefits will be taxable so you want to be protected against a huge tax bill. As Anonymiss said, you should be provided with tax assistance.
- home visits. At least one annual trip home for the family, and 2 extra days of holiday as travel days.
- foreign currency exposure. You will encounter fees for transferring money. Also reasonable to protect his salary for exchange fluctuation. The pound is on the rise and you want to be protected if his salary is still based in USD. your buying power could drop dramatically.
- temporary accommodation for at least 30 days. Assistance in finding a property to rent and assistance with getting deposits in place (you won't be able to open a bank account until you have a lease)
- shipment of at least 2000 pounds of goods for your family (and the return at then end of the assignment)
- a disruption bonus equivalent to two months salary on both ends of the move
Will be very lucky to receive:
- COGS adjustment for cost of living increase
- housing allowance
If I'm honest, to be like-for-like, you would want the US salary equal in GBP. So if you make $50k in the US, you would feel an equivalent buying power with £50k in the UK. Will you get that? Unlikely. Salaries are typically less in the UK (you adjust). For me, I took an international assignment here and my disposable income disappeared and I used a lot of savings. But what I lost financially, I gained in experience and now I earn far more than if I had stayed in the US. I have a friend from the UK on an international assignment in NYC right now and he can pay his rent and daughters tuition from his salary, everything else is debt he is incurring. But he knows at the end of the assignment he will be rewarded. It's a trade off.
Also you may need a back-out plan. I saw you have not been to the UK before.... It's not for everyone. You need to know you can go back earlier if needed. You know how everyone says it rains all the time here? It's because it does! It can be very hard to adjust.