Skip first leg of train??

[mondaytosunday]

I'm going to Exeter from London tomorrow. I booked through SWR but the Paddington to Exeter leg is GWR.
So, the site booked us Wimbledon to Waterloo on SWR, tube to Paddington, then Paddington to Exeter on GWR. I've just realised I can go direct to Padding from Wimbledon by tube. I will be collecting the tickets from a machine before travelling. My question is: can I bypass Waterloo and just take the tube direct to Paddington? Will my ticket be invalidated in some way if I do this? I get free travel on the tube so could not use the ticket at all til I get to Paddington (I did check out fares from Paddington to Exeter but it was more expensive)! This is what makes me hesitate - will the system know I didn't go through Waterloo and cause a headache? I've got booked seats I have to get on the exact train at Paddington. Tickets were £££ and non refundable.

You'll be fine. My husband works in the railway industry, ticketing specialist and he says don't worry. Just use your ticket at the barrier at Paddington.

Thanks! I don't know why they had us do such a roundabout circuit. I thought it might be Saturday closing or something but checked and TFL says Wimbledon to Paddington that day.
Now my worry is it says to collect ticket from machine using card I used to pay for it - I'm sure I used Apply Pay but don't have an email to check - thing is my debit card associated doesn't work in machines as the back is worn! 😬.