How much to plan for gas for a two week trip to England, a guestimate would be wonderful

Go to www.viamichelin.com and punch in each of your trips. It will give you distance, time and fuel costs. For example, London to Exeter (Devon):

Distance: 279 km with 132 km on motorways
Time: 03h35 with 01h21 on motorways (not including any stops)
Costs: Petrol 29.49 GBP

(BTW cars run on petrol in the UK, not 'gas', except this bus).

Looks like you may nerd to challenge your arithmetic skills.

Estimate probably distance traveled - point to point to point plus driving around.

Estimate fuel consumption rate for city-highway mix anticipated.

Guess fuel price for future and adjust for regional differences
http://www.theaa.com/motoring_advice/fuel/

Unleaded prices are up 3.6p from 108.3 ppl to 111.9 ppl. Diesel
prices have risen 3.1p from 115.1 ppl to 118.2 ppl. The price
difference between unleaded and diesel has shrunk to 6.3 ppl.

Multiply. Then bring more.

Thank you so much for the Michelin trip planning tip. Went there and figure it all out. Thanks again!!

It depends whether you rent a diesel or petrol car and how many miles per gallon it does. (Tell the Brits you want 'gas' and they will think that your car is powered by hydrogen or natural gas - you must use the term 'petrol' or 'diesel'). Try and buy at supermarkets on the edges of towns or cities as they tend to be cheaper. Buying on the motorways (Interstates) will be more expensive.

I filled up today (diesel), and it is certainly the case that the prices are creeping back up after falling for a while and then steadying.

Be aware that prices around the country, particularly in the rural areas and on the motorway can vary considerably.

I'm curious about this part of the plan:

London for (3) nights (will park car there and not drive it)
Will drive to attend a wedding near Cambridge (1) night, then will drive back to London, return the car

Unless there's a special rate involved for extended rental period, it seems to me you'd be better off returning the car after York and then renting one for two days rather than pay rental and (presumably) parking for your three days in London.

London. Take care where you drive. Locals we met for dinner in London realized that they were entering a congestigation mitigation zone and were able to use their cell phone to pay the entry charge. If you don't the rental car company may send you a deferred bill.
http://www.tfl.gov.uk/modes/driving/congestion-charge
I'm sure Nigel can explain this process better than I can.

Much of what you plan can be done by train. It would be easier to go York > Cambridge (wedding) > London by train or car.

A car is a hindrance in Bath & London but would be useful in the Lake District. I don't get why you would go all the way to Devon & then ignore the south-west (Cornwall etc.).

www.nationalrail.co.uk (fares are cheaper on long trips when booked in advance).
http://www.theaa.com/route-planner/index.jsp