Manufacturing is important, but is it the first question?
First, you need market analysis and research to quantify the opportunity.
Next, you must look at sales and distribution and understand, for instance, the questions that buyers in major retail chains will ask. How will your idea translate into value for them? How will you get your first test market?
Then, if your product (its formulation, its production process, its physical characteristics) is going to need certification on the one hand or protection on the other, you need to get this realistically costed and time-lined.
Next, how are you going to obtain samples or mock-ups of the product and appropriately designed packaging, and what about the first production runs? The UK isn't always the best place for volume manufacturing, but that isn't your immediate question. For now, you should be looking at contract manufacturers and product designers. As the UK has great quality and choice in both, start with domestic options. That will simplify communication and co-ordination, and de-risk the early period which is expensive in time and cash.
All of this presupposes that your best route is to do the whole thing yourself. For many innovators in consumer goods, the most effective route to market is either partnership with manufacturers or distribution channels, or selling the idea in to established brands. These are additional options you may choose to explore.