I'm a big fan of gift experiences. They are always well received, and you get the flexibility to choose which venue and date... so it's a bit more focused than gift vouchers.
They do them in a wide range of price points, so there is something for everyone. I got a cousin a tea for two for her 70th birthday, and she was thrilled - took her hubby out for afternoon tea... not something she would have thought to do herself, but a very pleasurable thing for a special occasion.
Then on the vouchers front, I would perhaps do vouchers with a personal shopper appointment to make it more special. M&S don't, as far as I know, have a personal shopper service, but many big stores do (Debenhams, John Lewis, House of Fraser) and so do some shopping centres.
Personally I don't like giving cash, but vouchers are very useful and flexible. The younger generation that I buy for invariably will get Amazon vouchers so they have an extremely wide range of items to choose from.
It's great if you can hit the jackpot with your shopping and find the perfect gift for someone, though.