There are two (arguably three) separate aspects to the question as to what is legal after Brexit.
1. As to whether there will be a period of grace, under which existing reciprocal arrangements will stay in place. Nobody knows the answer to that yet, nor how long any grace period might be
2. That the intermediary dealing with the business from outside of the EU (for example a UK registered broker) is able to still deal with business (risk and exposure) emanating from within the EU, for example a UK registered intermediary dealing with Spanish compulsory third party motor liability. There are several ways by which an intermediary can become able to do this. The same applies to an EU domiciled intermediary seeking to do business into the UK
3. That the insurer is themselves authorised to deal with EU exposures from the UK and visa-versa
The important thing to note is that the duty to be EU / non-EU compliant cuts both ways.
As regards point 3, the vast majority of the large motor insurers (Aviva, Axa, RSA, Equity Red Star etc etc) have already confirmed that they are EU / non-EU compliant. Many of the largest intermediary companies (brokers, for example) are similarly ready; I work for one.
As regards points 2 and 3, it has come at a considerable cost: financial, legal and time consuming
We now only wait to hear:
A. If Brexit happens
B. If it does, at what terms
C. What the future looks like thereafter
There is nothing useful bods can do in the meantime; so don’t worry about it. I don’t.