Your mechanic should be able to figure out the correct length - otherwise I wouldn't trust him to build a good wheel... Other than that: I think you are fighting a problem that doesn't exist. Brompton front wheels have been radially spoked forever and nobody seems to complain about a front-wheel collapsing. The Brompton is specified for a 110 kg rider plus 2*10kg of luggage - should be enough for most people. Many people (including myself) have been using the Brompton in trolley mode for years w/o a single problem.
If your spokes hit the frame you may have a different problem that you will probably not get rid of with crossed spokes but by searching for the root-cause: Where do the spokes get contact, when and why?

What I've seen once is the front-spokes coming in contact with the axle-nut of the rear wheel if i.e. a S/A 8-speed is fittet that uses wider axle-nuts.

If you want to respoke your 2015 front wheel non-radially you should maybe think about a new/different rim as well, otherwise this may be counter-productive - starting from 2013 on front- and back-rims of the Brompton are drilled differently (front rim for radial spokes, back wheel for crossed spokes). Easiest thing would however be to get a SON-classic-dynamo-hub-wheel. This has crossed spokes plus it deliveres decent light.