Ok then lets spend all those tax pounds
Ok, If the answer is the system must cope with the maximum possibility as many of propose then they will have to build a system and the infrastructure to cope with every single tax return being submitted at exactly the same second, anything else has the possibility of not being able to cope.
Would you rather they spent all those taxes on something useful or invest it in a huge computer system just on the off chance every one in the country wants to submit their return at 23:59 on the 31st Jan which judging by your comments you think should be possible.
I think you need to take a reality pill.
Before online filling there where issues with the post because people posted them too late (and the posties went on strike or the revenue did) and then there was the saga of the revenue having to employ people to come in specially to empty their huge letter boxes because every posted them by hand at 23:59 and they overflowed leading to them being nicked for all the nice information contained within them.
Nothing has changed and they clearly some people don't learn by their mistakes and think to not leave it until the last second.
If you run the deadline you should expect problems. The only real answer is to allocate an even distribution of tax return dates through every day of the year, this would be much easier for accountants and the revenue.
Only problem is then people would moan it wasn't a memorable date, they hadn't been reminded by press and tv etc. etc.
I hate to take the revenues side on anything but I think you need to take some responsibility for your own actions. its no wonder we have a nanny state and are quickly heading for a 'super' nanny state.