A lot of eBay sellers are 'drop shippers', who never see a single product that they (claim) to sell.
https://en.wikipedia.org/wiki/Drop_shipping
So, while they might appear to be in Birmingham, the products you order from them could be coming from Beijing, Barcelona, Brno, Budapest or anywhere else they happened to have got drop shipping arrangements with. All you can do to avoid dealing with them (if that's what you want) is to look for all the clues on eBay, such as Chinese (or other foreign) names and/or addresses and to seek out sellers who specify the couriers that they use (such as 'Royal Mail 48' or other UK-based services, like CollectPlus).
Be warned though: UK couriers currently have some very long delivery times anyway, so you might find that, irrespective of where you order from, you'll have a long wait for your eBay item to arrive.
I ordered from a reputable UK-based eBay seller (with many thousands of sales and a 99.1% approval rating) on 14 April, with the item being despatched by 'Royal Mail 48' (which is the commercial equivalent of '2nd Class Mail') on 15 April. The package (which had clearly been posted as stated) took 14 days to get here, arriving on 29 April. I've seen other posts online that sugggest that, at the current time, it's not at all unusual for items sent by Royal Mail to take several weeks to arrive.