I don't believe there are any varieties resistant to carrot fly, only less attractive (lower odour at a guess). The only way to avoid fly is through protection using fine mesh or fleece.
I agree. After years of trying all sorts of methods.. tall wall of mesh which blew down, growing alongside onions, which not only didn't work but the onions didn't enjoy sharing their space, marigolds, and so on, a nice neat, tightly stretched mesh tunnel is a guarantee of success. I bought some at a very reasonable price from an Amazon seller in Hong Kong... arrived in 10 days , 2.4m by 10m for about £16 inc p&p. Goes through the washing machine at the end of the season too.
(Liked it so much I covered my cabbages this year too and had perfect heads for the first time ever)