No, they're perfectly accurate, you're just deluded in what you think they're supposed to measure.
It's your internet connection that cannot support 100mbps when it is busy, not the speedtest server.
Lancaster gives over 500mbps at peak times even with the umpty-dozen other tests it is doing. Manchester does over 350 and Maidenhead over 300.
Here's a list of nearby Speedtest.net servers that can do over 100, all tests were done at peak time.
Code:
Server Country Download Upload Ping 20/30mb 50/60mb 100/120mb
Lancaster UK 520.7 278.9 5 Yes Yes Yes
Luxembourg LU 304.8 226.9 18 Yes Yes
Massy FR 297.2 225.4 21 Yes Yes Yes
Manchester UK 246.2 158.8 11 Yes Yes Yes
Contern LU 235.4 211.0 19 Yes Yes Yes
Naaldwijk NL 220.6 161.0 25 Yes Yes Yes
Malmo DK 207.8 149.1 34 Yes Yes Yes
Merzig LU 196.8 157.2 31 Yes Yes Yes
Steinsel LU 196.7 228.2 19 Yes Yes Yes
Mechelen BE 192.9 182.6 27 Yes Yes Yes
Alkmaar NL 184.9 160.5 26 Yes Yes Yes
Clichy FR 183.4 190.1 19 Yes Yes Yes
Eindhoven NL 167.7 170.8 28 Yes Yes Yes
Bonneville FR 163.5 160.2 30 Yes Yes Yes
Aubervilliers FR 158.9 211.6 21 Yes Yes Yes
Paris FR 115.3 127.5 22 Yes Yes
Lyon FR 106.3 175.5 30 Yes Yes
Cappelle Ij NL 364.5 36.4 21 Yes Yes Yes
Maidstone UK 277.8 57.6 11 Yes Yes Yes
Kingston UH UK 262.0 36.9 20 Yes Yes Yes
Rotterdam Qweb NL 248.5 51.5 20 Yes Yes Yes
Capelle NL 243.2 51.5 31 Yes Yes Yes
Roubaix FR 205.7 46.2 15 Yes Yes
Randers DK 188.3 54.3 39 Yes Yes Yes
Bournemouth UK 167.4 26.2 13 Yes Yes Yes
Veendam NL 167.2 16.6 22 Yes Yes
London UK 138.0 36.7 11 Yes Yes Yes
Dublin Eircom IE 137.7 16.6 30 Yes Yes
Ashford UK 128.3 42.3 12 Yes Yes
Sittingbourne UK 123.0 51.2 12 Yes Yes