So how to ensure that every employee has a good time, and can stay for the full duration of your event? If you have a boardroom (or another large area: warehouse, classroom, sports hall etc) and basic facilities, why not host the event at work? There is event planning budget software that can help you with the planning, keeping you on track and ensuring you do not forget anything.
In this way, you can provide food and drink for a fraction of the cost, getting staff members to volunteer to help with the prepping in the hour or two before it begins, and have an informal buffet.
This way the food is all fresh, and you will easily be able to cater to any idiosyncratic dietary requirements, and you can let your staff bring a plus-one. This will break the ice between office groups as partners will chat to everyone, and you and your colleagues can put names to faces from amusing anecdotes. You can even get everyone involved with cocktail hour ideas for party games: cocktail making, charades, and so on. Games like this are much more fun when the general public is not sitting in judgement of your party!