According to this posting on the BBC's R&D blog, BBC HD in Freeview is now broadcasting HD in both 1080i and 1080p:

A great deal of material is shot natively at 1080p25 ... Within a single programme interlaced may be used for moving credits, cross-fades and studio shots whereas progressive may be used for location shot material. Up until now ... 1080p25 material was encoded using a 1080i25 encoding mode. But after a software upgrade on the 22nd/23rd March the HD encoder supporting BBC HD on Freeview HD has been set-up to automatically detect progressive material and change encoding mode appropriately. The encoded bitstream can only change at each GOP boundary to ensure decoders maintain a consistent display.

According to this thread on DigitalSpy, the encoding change shows up on the programme information displayed by a Digital Stream HD PVR. I have checked a couple of recordings from BBC HD on my Humax (including one which was definitely reported to be using 1080p25 by DS users) and can see no change on the iPlate during the programme. However, I detected no adverse impact on the picture or sound (unlike Sony FreeviewHD TV owners, apparently) so this looks like a fairly minor gripe.

The thing is though, I tend to keep my Humax set to output 1080i, on the understanding that this would upscale SD programmes and pass through HD programmes (which up until now has always been 1080i) pretty much unprocessed. If there is 1080p source material now coming available, would I better switching my box to output 1080p? What effect might that have on programmes still broadcast in 1080i?
The simple answer is to try it. SD looks soft whatever you do so perhaps try 1080p to see if you can see the difference on the HD material.

If they are switching between 1080i and 1080p are they suddenly doubling the bit rate or compressing more heavily, something has to give.
Sorry, not to my eyes, on my setup.

Softer than 1080i, if it doesn't then you're not gaining any benefit from HD. A good upscaler can provide a good picture but it will neer be the same. Of course picture size and viewing distance may mask the differences.

From the link ChrisDaniels posted: 1080p25 is a compromise format which uses more or less the same bitrate as 1080i50.

For some reason I was thinking of 1080i25. 1080i50 will have an almost identical bitrate at the same compression since the numbe of pixels per second is the same.