what time to leave hotel

Hello peeps,we're flying out to St lucia next Sunday
and will use the twilight check in the night before.We are staying at the Gatwick Hilton which has a walkway directly to the terminal.
We want to use the duty free after security.The gate closes at 8 in the morning.What time should we leave the hotel at to do all this?[:?]
How long to get through security do you all think?.Any replies appreciated.Thanks,Katie

We want to use the duty free after security.The gate closes at 8 in the morning.What time should we leave the hotel at to do all this?[:?]
How long to get through security do you all think?.Any replies appreciated.Thanks,Katie