Gliding advantages and pitfalls
Do it, but tell both instructors. Gliding is very good for you. You will become comfortable with all kinds of things that most PPLs simply don't do, will learn to use your feet properly, and will become very much more comfortable with various engine failure scenarios. You will also have lots of fun and become more fit and healthy from pushing gliders around outside all day.
Your basic handling will come on a treat - there is no such thing as straight and level in a glider, you spend hours adjusting various kinds of steep turns in bumpy air. You will learn a lot about manoeuvring right up close to the stall for long periods of time, with no stall warning horn. You will eventually be able to recover from an incipient stall and/or spin from a turn without even thinking about it.
You will find that the checks are all different, that the controls seem very light and powerful (except perhaps the ailerons) , and that you move on the air rather than through it. You will also develop an appreciation of magnificent cockpit field of vision, and this may well spoil you for high wing Cessnas or any other type where it's hard to see out.
You will learn a lot about low-altitude English weather, and it is worth it for this if no other reason.
On the other hand, you should probably watch out for two possible syndromes.
It's possible to become slightly blase about certain practices in gliders that would not be at all sensible in a most powered aeroplanes. This applies especially to the heights at which it is safe to carry out stalls, steep turns, spins and other manoeuvres including aerobatics. My original glider training at 622GS RAF Upavon involved us, as complete novices, full-stalling at down to 500 feet in the circuit. You wouldn't do that in your Group A aeroplane. I'm quite happy to spin a K13 a turn or two from 2000 feet but with a powered aeroplane I look for recovery to be complete by 5000 feet agl.
When coming back to power, watch out for an undesirable habit I noticed in myself, having quite a bit of solo gliding before my PPL. That is a tendency to commit to landing. That's exactly what you must do in a glider, of course, but I found in the PA-28 once I was on final, if anything went wrong (someone on the ground occupying the runway, a nasty gust, a bad bounce) my natural reaction was to sort it out on the ground rather than in the air. In most such situations in a power aeroplane, of course the correct action is to open up and go round again, and indeed I now remind myself "Go around if necessary" as part of my last-ditch short-final checks, along with Prop, Undercarriage, Flaps.
If you are an experienced power aerobatic pilot, please don't try aerobatics in a glider without very thorough dual instruction, or you risk being way past VNE with the wings falling off in no time. Glider aerobatics have significant complications - it's not so much the lack of power as other factors e.g. the minimal drag, the way the airbrakes usually reduce the G limit when deployed, and the lack of slipstream over the tail surfaces at very low speeds.