If the current lever are original then this is a bit curious; Those brackets are overkill for a band stop (normally just a little piece of steel plate on the underside of the down tube). What is it that makes you think the levers are original? Do you have pictures?
Upon doing some googling, i've found that lots of the Raleighs were sold originally with Huret friction shifters, which are the ones found on my bike.
It looks as if some banded shifters might be able to sit on top of these tabs and act as the stop at the same time. On the band shifters that I have currently have, only one side will sit on the tab, but simply isn't wide enough to reach to the other tab.
Hi all, thought id share some photos of the bike in its almost complete state. Needs the decals to go on and the steed will be finished! Although I cant promise I wont be tempted to keep tinkering. I will probably change it over to a Simano shifter set at some point. Been out on a few rides and it feels lovely, the downtube shifters take some getting used to but im sure it will come in time.
Nice bar tape. You may find gear changing easier if you mount the levers above the bosses. They should pretty much fall to hand as you drop your grip from the bars and with a little practise it becomes second nature to be able to change both together if required (thumb on the down tube, 1st finger on the front mech lever, middle finger on the rear). Those Simplex levers aren't the smoothest but I respect your desire for originality. Simplex made a system called "Retroshift" (??) that incorporated a spring within the lever body that balances the mech spring and means only a very small amount of friction is needed. They sell for a small fortune now (possibly more than the bike is worth if you find the wrong seller) but they are a joy to use and are the same brand. Alternatively the ubiquitous Suntour Powershift is a great (if a little heavy) beginner upgrade.
Shifting is a bit of a chore at the moment, and as you say the current Huret shifters aren't a dream to use. Might have a look at some suntour as you suggest, or maybe even go over to indexed. ** Rather than indexed I mean ratcheted.