In the circumstances, the old rules are probably the most reliable. Given the time of year and location, expect rain so you need jeans, simple sweater, probably thin cashmere, proper walking waterproofs and flat sturdy boots you can go anywhere in (I would pack a pair of gloves and a flattering beanie too).
Then I'd look at Me&Em for a dress-up/dress down dress for Saturday dinner that you can re-style for smart work days and weekend fun time events, and I would also be inclined to buy some luxe lounge trousers for Fri evening. Me&Em are good for these. I like the satin cargo trousers.
Do the rest of the styling with accessories from your wardrobe and some cheapie high fashion ear rings, but no bling (other guests may bring real diamonds); big costume pearls are great, often cheap at bric-a-bracs and very, very on trend. A tidy small cross body bag in a contrast colour to your scheme (I adore the Geminis bags from Parfois, because they have four colours per pair of tiny bags-- they clip together): at £19, it's not an investment, but mine has stood a year's almost daily use and is still pristine). Stick to a colour scheme, just ONE neutral-ish colour scheme for the whole weekend and be ruthless, depending on what suits you best and feels comfortable. Toiletries, warm PJs and a good read of the newspaper for a few weeks ahead of the event and you should be good to talk to anyone and do anything. Hope you have a lovely time.
PS. I do hope I have pitched this approximately at your budget.