What Pomp does, I have seen done by others too. And it seems to be held as the absolute best and cheapest way to convert money for use abroad.
The Post Office might have no fees, but the spread is horrible. So say on the US$, if the market rate is $1.95 to the pound, from memory the PO rate would be something like 1.85 (sell) - 2.05 (buy), a 'spread' of about 20 cents. Whether you buy or sell, you are losing around 5% (.1/1.95), and that is on a very 'liquid' currency. The spreads on currencies like the Egyptian pound are much worse something like 20% in total!
p.s. I changed some money from Sing dollars to US$ a few months ago, at a money changer in Singapore (US$1 = S$1.5 or so). The buy-sell spread was under half a cent, from experience this is normal there, and about a 0.25% spread. In fact, with a bit of shopping around I was astonished to find a changer who offered me better than the live money market rate (another changer I've known for many years, showed me the live rates on his Reuters pager, and suggested the guy might quote the rate he did, but when I returned to him, he would not actually deal at that rate... well, he did)! He must have had a position in US$ that he was keen to clear.
In a Tokyo, I've sold yen for US$ in Citibank for under a 1% spread
In Egypt I've bought EL for GBP for around a 2% spread.
I have yet to see a competitive UK money changer. In fact I cannot think of country where the rates on travel money on the high street are so bad.