There is currently no central guidance about this. In general, adoption agencies are advising against any direct contact. Postal contact is ongoing, though it is not likely to happen in the previous timescales due to difficulties with handling post, buildings being shut and other lockdown restrictions. 

One Regional Adoption Agency, ACE, is using this guidance:  

We would encourage adopters to get in touch with their RAA to let birth parents know that the children are well, they are being looked after and that the advice regarding social distancing is being followed. Birth parents are encouraged to do the same.