Hmmm...
I get my GRW chicken-hopper tanks from SIG Mfg., they have never been out of stock. Surely SIG products, and the goods they distribute, are available in the U.K., even if not next week.
My guess would be the 3.75-ounce unit will in fact fit the ARF Flite Streak, although it is possible it would not drop in on a model which has been fitted with another tank.
My most recent ARF FS has, of course, a 3.25-ounce GRW tank, but I moved the motor back a lot--about 1/2", even had to cut into the fuselage sheeting--and the tank fits with no problem, even if there is not much room to spare.
Unfortunately, 3.75 ounces is quite the fuel load if using a 20 or 25.
Second thoughts: If all I had was a 3.75-ounce tank to work with, they can be shortened fairly easily. I have even considered doing a chop job on my 3.25-ounce units, just ran out of motivation.
Incidentally, the motor--O.S. 25FP--is shoved back in my model simply because I was determined to use the stock muffler, did not want to add lead to the tail.