You will be filing direct onto the system without the human intervention you get at the moment. It will probably result in most VFR flight plans being rejected.
Fat chance of that happening, and an even slimmer chance of anybody checking anything in a wholly-UK VFR FP.
You can already file flight plans via www; have been to for years. There are several national websites;
Homebriefing is the best one which handles flight plans to and from anywhere, for Euro 36/year.
IFR (airways) flight plans should never actually get lost, provided you depart within the time slot. They are distributed to the airways sectors within seconds of acceptance by the above website. Homebriefing also sends you a confirmation of acceptance (note: this is
not a confirmation of correct distribution; there is no such thing) by SMS and/or email. I find their SMS usually arrives far too late, but the email is instant.
VFR FPs sometimes get mis-addressed especially if one or the other end is a field which is not on the AFTN in which case the FP goes to the nearest "proper" one. Sometimes it is sent by fax to the airfield; I don't know how this is done.
The answer to lost flight plans is to phone the dep+dest airfields before going. VFR FPs are often lost; it's the nature of the system.
A good way to get them lost is to put a DOF/ on it a few days in advance...