I have an Arrow and found that the front brake is not nearly as bad as I feared from various reports.
Some points to check:
1. Make sure that the steel top hat bush in the backplate for the spindle is not badly worn as this allows the plate to flop about. I made a new one for mine.
2. Check the routing of the front brake cable which should go across from the handlebar lever to the left hand hole in the dummy tank then cross in front of the steering head. When I bought my bike it went down the right hand side (which is where the clutch cable should be).
3. The handlebar lever should have 7/8" pivot to nipple distance. Many have the incorrect 1.1/16" levers (inclding Arrows at the NMM).
PTFE lined cables are always a good idea. I generally have brakes relined by Villiers Services
http://villiersservices.co.uk/index.php?main_page=product_info&products_id=146 but the existing linings on my Arrow were fine so didn't bother in this instance.
Have a look at the Yahoo group
https://uk.groups.yahoo.com/neo/groups/arielarrowandleadergroup/info as well as the Ariel Owners
http://forum.arielownersmcc.co.uk/index.php?sid=7f6ae8f94ea43c61246eca8d0c03b805 as there are some knowledgeable people in both places and I know some have fitted TLS brakes. But don't expect it to be easy.......