I'd much rather buy a driver, but there are no 17mm (or any other size) drivers that can support a Vf of 8V up to 700mA. Unless you know of one. I'm working with high Vf UV LEDs here.
I understand. Sometimes there just aren't any ideal choices. Not an expert but I looked around and didn't see any drivers from the usual suspects that would work - taskled.com, ledsupply.com, etc.
Sometimes that is a useful indicator that the challenges of building something with a nearly 2 amp input boosted to 8 volts output isn't so trivial to accomplish in that size package. (found some that are larger at taskled.com)
Boost drivers can be tricky to manage - and if there is not a load can go into runaway and destroy the output components.
Sometimes there are applications limitations - for instance if the chip company doesn't think that a Vf of 8 volts is very likely, they just won't design for it. The chip designers are typically going for the volume applications, such as USB, computer chip power supplies, and conventional LED drivers.
Unless you go to fairly high frequencies, getting the inductor much smaller than George did on that taskled board will be tough.
If somehow you could come up with a battery configuration so that Vbat > Vf, life will be much easier. A single 18650 isn't always the only possible solution. 3 x R123s or 3 x 18650s ?
It might sound ancient, but for a (3 x R123) + (your UV LED), the efficiency of a well chosen, simple resistor circuit might be (efficiency) competitive with a high tech boost circuit.
Resistor based circuits are also quite robust. From a past project I think I still have some 2 and 10 ohm high wattage resistors built on a high conductivity substrate - think it was AlN but need to check. Let me know if that would be useful. It has been a while, but I think that they are 1206 size but can dissipate a ton of heat.
Looking at your past posts I realize that your electronics background is way past mine but for fun:
Assuming a nominal V bat of 3.8 volts x 3 = around 11.4 volts V bat
Vf = 8 volts
11.4 - 8 = 3.4 volts
RI = V
3.4 volt / .7 amps = 5 ish ohms
So a pair of 10 ohm resistors in parallel will be pretty close if you are willing to be slightly imperfect.
8 volts x 0.7 amps going to the LED
3.4 volts x 0.7 amps going to the resistor
I guess not so ideal but simple.
Harry