Collecting signatures for the General Election is usually straightforward - but there can be difficulties. Agents must ensure that everything is in order before the forms are submitted. The Electoral Registration Officer will advise on any problems - but the deadline must be met. For that reason most nomination forms will already have been submitted. In a local election a few years ago (where there is also a 10 subscriber rule), I agreed to stand as a 'paper candidate'. I got the 10 signatures - but one of my subscribers forgot to tell me she had also nominated the Conservative candidate. When my forms were handed in (I wasn't around - as I had to rush across the country to where a relative was very ill) - the list was rejected because the subscriber had already been accepted as a subscriber for the Conservative candidate. There was no time to get a new list - and so I couldn't stand!
The Electoral Commission has produced a detailed book for candidates and their agents on the process. It is available here.