Does a Sky HD digibox or a Freesat HD set top box really improve UK TV reception in Spain?

Yes. To an extent.
For BBC 1 HD, BBCHD, ITV1 HD and C4HD, the signal from the satellite is stronger and available for longer than the satellite signal standard definition versions of the same channels.

This can mean that if you are losing standard defintion BBC1 regions early in the evening, then the HD channel should be available 24/7 (of course this depends on your dish size and location.)

Sky HD Channels: In some areas the standard defintion versions of some Sky Movie and Sky SPorts channels can be restricted during the afternoon.

For example it is possible that Sky Sports 4 and Sky Sports News are not available for about an hour either side of 5pm, even on a 2.4m satellite dish. However, the HD versions of Sky Sport 4 and Sky Sports News are available 24/7, even on an 80cm satellite dish!