Overflying instrument approach tracks
 

Traffic above Cambridge could be at any height if they're practising holds.

But it's class G, so you don't need permission. Talk to them, and if there's a problem they might politely ask if you mind sticking to your current altitude so they know what you're doing and can keep the instrument traffic out of the way, that's the most likely outcome. You don't need to talk to Duxford until you're clear of Cambridge.

If this was about decent aerospace, the answer would be simple: in any controlled area, coordinate with the controlling authority. In class G, apply whatever the AIP says, and it never hurts to request advice from FIS.

But as I understand it, the Brits love to make things unsimple...

Are you talking about the ILS track on a chart? The gradient is usually about 300' per mile, so if there's someone on the slope, that's where to expect them. If you're VFR in uncontrolled airspace, just look out the window like normal - you can fly down it, across it or even up it - but common sense says avoid it especially if it's the active.

At 10 miles, it's around 3000' so you could fly under it at 1500', likewise at 3 miles it'll be around 900' so you could fly over it at 2000'

If more or less overflying the airfield, then the inbound leg - the one marked on the chart! - is indeed low enough not to have to worry about; it's the beacon outbound leg starting from 2,000' or 3,000' that's more likely to be interesting. Oh, and if Cambridge ATC is on one of its half hour breaks due staff shortages there will be people milling around at all levels in the overhead waiting for them to open again, but IME the "Cambridge traffic" calls work fine.

Suggest overflying Cambridge at 2,500' - that's what most passing traffic seems to do, cloud permitting. I've never heard this be a problem (well, instrument training traffic might be given "not above 2,000' on the go-around" to keep out of your way, but that's all good practice for the student).

Unless you are in a warbird, the circuit is to the south of Duxford airfield so it is easier to route in via the town of Royston, which Duxford like to use a VRP anyway. That helps keep you clear of Cambridge.

PS,
You probably know this already but, just in case, if you want to get to Duxford early, you need to PPR the day before. They don't open until 1000 and that's a bit late if you want a full day there. I nearly got caught out by this but they kindly let me in as I had been there before.

I didn't know that! So thanks for this.

It's a good question but a tricky one to answer. For typical conventional procedures, you'll cross both the outbound and inbound legs. As an example, the platform altitude for the ILS23 at Cambridge is 1700 ft, so you can expect inbound traffic to be not below that level until it has intercepted the glideslope at about 5 miles on extended centreline. On the outbound leg, which you'll also cross, aircraft may be descending to about 2000 ft from much higher levels, though most usually from about 3000 ft. If you talk to Cambridge Approach, they will be able to advise you as to any instrument traffic, and unless there's a conflict they shouldn't object to an early call to Duxford to out out your join.

You can find Cambridge approach plates in the AIP.

If you are west of Cambridge you may not be crossing the approach but the 23 departures. If jets they may climb quite quickly so there is no conflict free height/alt. They are friendly and really appreciate a call. If there is no conflicting traffic they will pass you to Duxford quickly.

Best bit of advice was from Gertrude - "talk to them". Simples.