A big event is expensive. Businesses arrange their advertising expenditure across a financial year and therefore making early contact with potential sponsors is important. You will want to identify a few different tiers of support that sponsors could engage with. The lowest should be accessible for almost any company (£100) and the highest should be high enough to make a substantial dent on your expenditure (£1,000+). A middle tier (~£500) is likely where many sponsors will want to come on board. At the bottom level of sponsorship logos will be included in all materials on the day and in advance. At a medium tier a presence at the day is likely to be available, usually a trade stand. At the highest levels you may want to let the sponsors stand up and speak to your captive audience for a few minutes.
Your institution may want to support your launch event. If you need financial support for this it will be worth negotiating while discussing pump-priming funding. Some intuitions find it easier to offer expensive venues for cheap/free than offering devolved research funds.
If all else fails you should be prepared to put some of your own research funds on the line here. If you don’t believe in the value of your launch why should anyone else. Hopefully, you won’t need this but being ready to contribute yourself will build a lot of good will.