I just use the mail app.
Its integration with the rest of iOS keeps me coming back to it. I’ve several personal email accounts on it and 1 work account. Switching between the ‘all accounts’ view and the work inbox during the week helps keep focus – just like skids and bigG describe.
During holidays or for long weekends I just go into settings–>mail & calendars, pick the work account and turn off mail and calendar and contacts with the switches. The mass of work emails disappear from the ‘all accounts’ view in mail and my calendar becomes less cluttered. Come the last day of the holiday I turn them back on and hope that I don’t have an 07:00 meeting invite for the first day back.
I have tried the outlook app (OWA). Having to open the app and enter a PIN made me think of GOOD for enterprise. I do not like to have to enter more credentials after TouchId-ing into the iPhone. The new OWA app has been judged insufficiently secure by our security folks and is no use at all to me.
[edit] you can use the mail app to get to your BT Yahoo mail can’t you? I use it for a Yahoo account that receives Flickr notifications. Or have BT made it ‘extra special’ that it doesn’t use SMTP/POP/IMAP and username and password?