I've never used "a few bob" ... I'm not even sure I know what a bob is!
As for "road tax" ... it's a myth that is perpetuated by the anti-cyclist idiots who need to justify their myopic view of teh world in which they have entitlements, because they're so special and much more important than anyone else.
If you really don't know -
A bob was a shilling
A tanner was 6d (pence) now 2.5p
A shilling is 5 pence (since 1971)
McDonalds is American and Americans never did understand British pre-decimal currency.
(Ice cream vans and the like used interesting counting coventions before 1971 with tourists...
Change was always counted up in days before electronic tills.
The pennies up to the next shilling (always called a bob to confuse them even more) ... 8, 9, [next] bob. Then if possible the next shilling was tanner, ninepence, [next] bob. Then, if the original total was over 10s accept going up to £1 as 20s, if less than 10/- count up to the 10, and call it a [decimalised?] pound (always a quid or nicker to obscure matters).
It was blatant fraud and needed careful targetting but was done by all the street traders. I helped several times on a friend's dad's ice cream van at a London tourist site in the late 1960s and saw it in action. The scale of the fraud was huge and the proceeds, much more than the pay, were shared out at the end of the day. Needless to say I, being scrupulously honest, never joined in with these despicable, discriminatory and illegal practices.
Some Europeans spotted it - particularly the Italians and French - but I don't recall American or Japanese tourists complaining.
I never heard of any prosecutions by Weights and Measures, but there certainly should have been.
The other game, which may still go on, was to let the Americans pay in dollars, which appealed to their conceit about their 'world' currency (a supply of US change was kept in a separate till drawer). The exchange rate was mind-boggling at about a dollar to the bob instead of about 4US$=£1! They were delighted and nearly always thanked the seller
for being ripped off for not having to cope with the difficult English money.)