Welcome to the forum! Roger Webb likes his 3018, you can find him on YouTube. He's a bit wordy in his videos but you'll see the 3018 put through its paces.
I have a Sainsmart 3018 Pro and it works great for small projects using Carbide Create CAD (free) with Candle for CAM. They also have an expansion kit to make it a 3040 for about $70.00.Thanks for all your feedback. I'm looking at the cnc option over the circular saw or simple router option because it would also serve as a hobby besides just making the lids. I'm just using the lid manufacturing to justify the purchase if I'm honest!
As for the spec, in hindsight I think the lids would be 14-18mm thickness as a maximum and the more realistic output expectation is around 150 per month but in batches of maybe 15- 30 in a day. I would also consider swapping out for pine if this would be easier to cut through than MDF.
I don't know if this all put together would make any difference if I increased the budget to £500 to get a better machine perhaps but that's the most I would want to put into this hobby I think.