Not an easy area.
The UK Met office does not release its 3D model (except to paying commercial users, a disgraceful tactic IMHO) - the only thing you can get is the standard SigWx form which shows indications of tops if above FL100.
A more recent newcomer is
Meteoblue which is a slightly tacky (Java based) site which can do slick looking vertical profiles from (mainly) the U.S. GFS model.
The GFS model itself can be accessed via numerous websites e.g.
NOAA but I don't think they have a facility at NOAA to plot tephigrams (a.k.a. ascent or skew-t) which is the presentation that is really needed for easy interpretation.
Historical cloud tops can be found at
Univ of Wyoming - these come from actual baloon ascents.
A really good site is
Meteox - this carries weather radar and anything red is a sure sign of high tops/CBs. This also carries an archive which shows the weather to be quite patchy on the date of the flight discussed.
If you PM/email me, I have more reading material on this.
None of the tops forecasts are accurate; it's a difficult thing to forecast, but used in combination with other general data e.g. the MSLP charts one can get a pretty good idea.