Standard package will include:
- Moving of personal belongings (should specific an amount and how it is being shipped by land or air)
- Visas for yourself and your family members
- Pets (sometimes, not always)
- Disruption bonus (usually 15-20% of your annual salary to cover miscellaneous costs, usually given both when you move to the foreign location and after you return home)
- 30 days corporate accommodation (you will not be able to do use this with pets)
- Assistance in obtaining permanent housing (the rental market is much different in the UK than the US. They should pay for a company to help you navigate renting, be your guarantor for your lease, assist you with deposits, opening a bank account, finding a local doctor, etc.)
- Private medical coverage
- Annual return trip home for you and your family (flights, rental car, possible expense allowance, two extra days holiday)
- Housing allowance
- Car allowance
- Tax assistance with both UK and US returns
- Guarantee for repatriation at the end of the contract
- International schools for children's education
Your contract should also be clear as to what happens if you terminate your employment prior to the end of the contract (will you have to pay anything back, etc.).
Also you mentioned they would pay you in GBP, not USD. As this is a temporary assignment, you need to think about your US situation. You *should* remain on the US books so that you continue to pay into social security to maximize payments when you retire. If they make you a UK employee, you will be paying into the UK retirement system, which you will have no benefit from.
You've also mentioned that you make mid-six-figures for a salary but they are only offering £1,500 a month for rent (which will not go far at all in London). A typical rule of thumb is that your monthly rent should be equal to 1/30 of your annual salary. So if you are making £150,000 a year, your monthly housing allowance should be £5,000 a month.
You will find places that are willing to rent with pets, they are just fewer and further between. When I moved, I came with two cats. My assigned relocation expert had vetted the properties and only showed me places that would accept pets. They are usually not high-end accommodation. I would say 1 in 10 places will accept pets so you'll have less choice, but you will still have a choice. Usually just a higher deposit is all that is needed.