Well perhaps I will upset you all in that my baby is 12 weeks old and I am going to start introducing baby rice in about 2 weeks!! However I wanted to make a couple of points.
Firstly the slow down in weight gain is probably related to her recent illness and as she has only dropped one centile is no cause for concern. Most children will drift between a couple of centiles at some point or other. Secondly (and very importantly), the centile charts in the red book are based on bottle fed infants growth, BF infants have a different growth curve. You can find the appropriate centile charts for BF girls/boys on the WHO website.
I am suspicious of all "expert" advice as I'm not quite sure who the "experts" are. Am I? No. As a doctor I feel just as unsure of my parenting abilities (and hate immunisations with a passion for my own child despite recognizing their necessary evil!!).
The guidelines to wean at 6 months come from the WHO and are guidelines for just that - the world. There is little evidence of harm by weaning at 4 months in the developed world, unless there is a strong history of serious food intolerance eg. coeliac disease. Most of us were probably weaned at about 4 months and most likely wolfed our food down. In the developing world (where formula is often still viewed as superior to breast milk) it is crucially important to delay weaning for a variety of reasons. Poor sanitation means that early weaning can lead to death, and the reduced fertility whilst BF is beneficial where family planning is not available/acceptable. Conversely, exclusive feeding after about 6 months is damaging as it does not provide all the nutrition a growing child needs. A friend of mine who is a paediatrician in a london teaching hospital with a large asian community tells me she sees babies brought in fitting because they are so deficient in certain minerals because they have not yet been weaned at the age of 1 yr!!
My reasons for weaning earlier are this, reflux - as a feed thickener I hope baby rice will cool his fire and therefore lessen his pain. Also, as I return to work at 6 months I want my DS to be happily weaned by me in a calm, loving environment before he has to cope with the abandonment he may feel by being cared for by a childminder. If I can get him to happily accept EBM and some puree during the day, hopefully he will not find the transition so hard in a few months time.
Anyway, I've gone on too much but mainly I wanted to say, I wouldn't worry about the centiles too much and do what suits you and your baby. Your baby will continue to need lots of milk even after solids have been introduced and breastmilk remains nutritionally superior for a long time, but that is not to say it doesn't need supplementing. As we all know, advice changes all the time. Hey once upon a time smoking was considered cool and consultants smoked on their ward rounds!! If your baby did like it, give it to her again! Just a small amount slopping around in lots of milk might excite her by the different texture and is a good introduction to proper foods later. If you don't want to, then don't!!