'surely' means undoubtedly, without fail.
It should be used in situations of 100% certainty.
• "Bus Stop M" ...surely the most written about bus stop in London?
• Surely someone at TfL looked at all those posters before they were given the ok?
• I think the most famous person to come out of Bewdley must surely be the lead man of Underworld, Karl Hyde?
• Surely that should read "If there are any further errors"?
It does not mean 'This is my personal opinion.'
It does not mean 'I haven't checked this, but hey.'
It does not mean 'Well that's what I'd have done.'
It should not mean 'You idiot, let me point out that you are wrong.'
If you need to add a question mark, the use of surely is probably inappropriate.
Consider instead using the word presumably.
'presumably' is used to convey that what is asserted is very likely, though not known for certain.
It should be used in situations close to 100% certainty.
• Presumably air pollution is always high in Central London.
• Presumably the TFL Rail link to Heathrow is now in zone 6?
• Presumably due to laziness the author never bothered to check.
• Presumably you wouldn't go to Ilford specifically looking for moth repellent.
It does not mean 'I have not considered any other options.'
It does not mean 'I have always assumed this to be the case.'
It does not mean 'I would have done this in the circumstances.'
It should not mean 'I think I know better than you.'
Consider instead using the word probably.
'probably' means that something is more likely than not.
It should be used in situations with a probability over 50%.
• The USA has Trump, we have Bojo. They're probably in bed together right now!
• If there was a general election called and they had enough candidates, the Monster Raving Loony Party would probably stand the best chance of winning!
• The EU will probably make us cough-up for a Green Card
• If you see smiling tourists pointing at buses with destination Penge they're probably Danish.
It does not mean 'This could potentially happen.'
It does not mean 'This is one of several possibilities.'
It does not mean 'I have no idea but...'
It should not mean 'I am making this stupidly unlikely suggestion as a joke.'
Consider instead using the word possibly.
'possibly' means that something could happen, it is not impossible.
It should be used in situations with a probability over 0%.
• This is possibly the first website I visit every day.
• W12 8QT was once quite possibly the most recognised postcode in the country.
• 'Brunel' is possibly the example par excellence for this.
• It's possibly to do with the extra length of the bus.
It's hard to go wrong if you use the word possibly.