By law, on orders under £135 (I think) the seller has to add on VAT and pay that to HMRC, which AliExpress does when you get to the checkout (as does eg ebay if you buy from abroad)
Over £135, the seller shouldn't add VAT but declare the value on the package and likely it will get picked up by customs with the end courier charging you the VAT plus an admin fee before you can take delivery.
I haven't ordered anything >£135 on Ali but some other sellers often 'forget' to declare the full amount and so usually nothing more to pay but YMMV.
EDIT: Actually thinking about it again the £135 threshold might be a Brexit related thing for orders from Europe although on small orders AliX does the same thing.
By law, on orders under £135 (I think) the seller has to add on VAT and pay that to HMRC, which AliExpress does when you get to the checkout (as does eg ebay if you buy from abroad)
Over £135, the seller shouldn't add VAT but declare the value on the package and likely it will get picked up by customs with the end courier charging you the VAT plus an admin fee before you can take delivery.
I haven't ordered anything >£135 on Ali but some other sellers often 'forget' to declare the full amount and so usually nothing more to pay but YMMV.
EDIT: Actually thinking about it again the £135 threshold might be a Brexit related thing for orders from Europe although on small orders AliX does the same thing.