The mainland sellers who decided to stop selling to the UK are those who bought into the anti-UK hysteria which dominates the media channels on the mainland. Like it or not, it is there.
And American firms have always been reluctant exporters. Too much hassle... there are firms in the US which do $10BN+++ sales just to the US govt, so why bother exporting? Getting US firms to sell you stuff can be like pulling teeth. And always has been. Many go to massive lengths to avoid exporting; a while ago I bought an unusual form of a BNC connector from a US firm. Value about $20, they would ship only through an "international fulfilment agency". Cost $250! Would they send it via the US postal service, for about $10? No way. Too complicated.
We do not go to the post office.And then they wonder why China is stealing their jobs by the millions. The chinks will ship anywhere, even if it involves faking their location on Ebay, to look local

Right now I am talking to a German company about sending something (big) to the UK. They want about £400 for shipping, while they ship for free to other countries in Europe

I even got a quote from a German shipper, £150, and asked this guy if I can get it collected. He isn't interested... Why do you think that might be?
I know I am repeating myself but
anybody on the mainland can still just simply ship something to the UK. IAW the way international trade has operated for centuries, it is
up to the UK end to sort out the import VAT, any duty (no duties applicable generally on EU imports, btw) and charge it to the end customer.
The fact that many customers hate paying import duties etc, feel ripped off, etc, is a separate issue. In my business we export a great deal to the US. For about 20 years we used to ship direct to customers there, with everything prepaid. The carrier (DHL back then) offered this as a service... not exactly cheap but very convenient to the customers. Now we sell there via a distributor who deals with this.