Exchanges that allow cash in, and crypto out are the exchanges that require the highest level of verification and now the approval process is taking weeks or longer due to the flood of people trying to open new accounts. Coinbase should allow you to purchase low amounts of crypto that you could then transfer to buy your Steem. The process seems very intimidating the first time, but it is actually quite easy, I would recommend transferring a small amount like $1-10 so you can master it, before spending large amounts.
If you don not already have an account that is approved to deposit dollars, sterling or other fiat currency. My recommendation would be to find a friend who already does and give them the cash for them to buy it for you. There are peer to peer services that will do this for you but the fee's tend to be high... If you only want to test the waters with buying a little Steem I would just find a friend that could buy $100 or so of ETH/BTC/LTC for you and transfer it your your Steemit account.
Goodluck!
That's really helpful, thank you. I've bought £10 worth of Etherium on Coinbase now and am just looking into transferring it to Steem. I think they're a bit overloaded at the moment, but your comments really helped.