Have braved the rain , walk round my garden sniffing my whites. Nicotiana ( usually treated as an annual) is in bloom for its third year. Its not fully hardy but if in a sheltered position survives winter. It is very pungent in the evening.
I have a Dictamus which is fragrent , mine is purple, but you can get a white version.
As Robinia suggests Sweet Rocket but cut back spent blooms for a second flush of flowers, then let seed.
Pinks and Carnations are available in the garden centres at the moment, some of which are very fragrant.
Sweet cicely, very pretty lacy flowers.
I have a large area Sweet Williams which include white all of which are fragrant, these I let ,seed themselves. Although the white hasn't got such a strong smell, it might be worth looking for.
White Lillies, its a bit late to plant bulbs but you may find some for sale potted .
Narcissus Pheasant Eye, smells heavenly and is one of the last flowering in the spring.
Hyacinthus -'L' Innocence