May 2021 update - I contacted Tesla UK for an update on the Model Y, and they were very tight-lipped (said ETA in the UK not available). Giga Berlin was due to come online on 1 July 2021, but has been delayed 6 months (Elon approved) due to planning/environmental/EU red-tape. Meaning that Giga Berlin won't be online until at least Q1 2022, which is when reservations for the Model Y in the UK are likely to open...
Our only hope (in the meantime) is that Giga Shanghai starts producing the RHD Model Y for the Australian/UK markets (they are already supplying the new 2021 Model 3 RHD to the UK). With demand in China being so high for the Model Y, the RHD cars are likely to be last in line for manufacture (as usual).
Without a miracle, the Model Y is a good year away from arriving in the UK (earliest Q2 2022).